How are the replacement effects of lands such as Breeding Pool applied?

Question

I am not sure how many replacement effects the rules text of lands such as Breeding Pool apply, and what options are available for applying them.

Suppose the active player controls an untapped Archelos, Lagoon Mystic and attempts to put a Breeding Pool into play. Which of the following accurately describe the situation?

1. Breeding Pool has one replacement effect, roughly stated, "Replace Breeding Pool entering the battlefield with either the player paying 2 life and Breeding Pool entering the battlefield, or Breeding Pool entering the battlefield tapped." Thus there are a total of two replacement effects to be applied to this event. The player chooses which replacement effect to apply first and which to apply second, and whenever they apply Breeding Pool's replacement effect, they also choose which of its two options to apply.

2. Breeding Pool has two replacement effects, but only one is immediately available. Breeding Pool's replacement effect to enter tapped is "unlocked" if and when the player applies its life-paying replacement effect and chooses not to pay. Thus there are a total of two or three replacement effects to be applied to this event, depending on the choice made for Breeding Pool's life-payment replacement effect. One legal option would be for the player to first apply Breeding Pool's life-paying replacement effect, choosing not to pay, then apply Archelos's replacement effect, and finally apply Breeding Pool's replacement effect to enter tapped.

3. Breeding Pool has two replacement effects, both immediately available. Breeding Pool's replacement effect to enter tapped, if it has not yet been applied, "disappears" if and when the player applies its life-payment replacement effect and chooses to pay. It's life-payment replacement effect, if it has not yet been applied, "disappears" if and when the player applies its replacement effect to enter tapped. Thus there are a total of two or three replacement effects to be applied to this event, depending on the choices made. The legal options for the player here seem similar to those in #2, but I believe they may differ in more complicated situations involving multiple players' replacement effects.

Answer 1

Breeding Pool has one replacement effect.

Breeding Pool's ability says

As Breeding Pool enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

The part that says "...you may pay 2 life. If you don't..." is a cost, as defined in rule 118.12:

Some spells, activated abilities, and triggered abilities read, “[Do something]. If [a player] [does, doesn’t, or can’t], [effect].” Or “[A player] may [do something]. If [that player] [does, doesn’t, or can’t], [effect].” The action [do something] is a cost, paid when the spell or ability resolves. The “If [a player] [does, doesn’t, or can’t]” clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred.

When you are applying Breeding Pool's replacement effect, you choose whether to pay the cost of 2 life. If you do, nothing happens, and if you don't, the land enters the battlefield tapped.

Answer 2

I did a bit of searching and found a few sources confirming that Breeding Pool's ability does create a replacement effect; the most authoritative is probably this post on the Magic judges' rules blog. It doesn't explicitly say that there is exactly one replacement effect rather than two, but it's hard to read that post in a way that suggests there could be two replacement effects. (Plus, given my own experience with Magic, I would never think that Breeding Pool's ability would create two replacement effects.) So I'm fairly confident stating that Breeding Pool has one replacement effect modifying how it enters the battlefield, and you just have to make a choice to determine what that replacement effect actually does. Therefore, your options 2 and 3 are incorrect.

Then, there's a ruling on Archelos, which you can view on Gatherer, that says

If another replacement effect says that a permanent enters the battlefield tapped while Archelos is untapped, the entering permanent's controller chooses whether the permanent enters tapped or untapped. If a permanent is simply put onto the battlefield tapped without a replacement effect being applied, it always enters untapped if Archelos is untapped.

Even though it doesn't explicitly say so, I believe that the reason the permanent's controller gets to choose whether it enters tapped or untapped is precisely because they get to choose the order in which to apply replacement effects. That implies that your option 1 is correct.

Depending on the player's choices, these are the various ways this could play out, if Archelos is untapped:

• Apply Breeding Pool's own replacement effect first, then Archelos's effect

  • Choose to pay 2 life

    In this case the event of "Breeding Pool enters the battlefield" (untapped, implicitly) is modified by Breeding Pool's own effect to "Breeding Pool enters the battlefield and you pay 2 life", then further modified by Archelos's effect to "Breeding Pool enters the battlefield untapped and you pay 2 life"

  • Choose not to pay 2 life

    In this case the event is modified by Breeding Pool's effect to "Breeding Pool enters the battlefield tapped" and then further modified by Archelos's effect to "Breeding Pool enters the battlefield untapped"

• Apply Archelos's replacement effect first, then Breeding Pool's replacement effect

  • Choose to pay 2 life

    In this case the event is modified by Archelos's effect to "Breeding Pool enters the battlefield untapped" and then further modified by Breeding Pool's effect to "Breeding Pool enters the battlefield untapped and you pay 2 life"

  • Choose not to pay 2 life

    In this case the event is modified by Archelos's effect to "Breeding Pool enters the battlefield untapped" and then further modified by Breeding Pool's effect to "Breeding Pool enters the battlefield tapped"

Obviously, one of these is distinctly better for you than the others. :-)