So from the sounds of it, you've just sat through a Game Design 101 course lecture, and part of the homework is to identify all the mechanics of the game. I had to do that too at one point.
The thing you're probably having trouble with is this: “mechanic” is kind of a broad concept! Essentially, a “mechanic” is every single thing the game does or every option it gives you. (Your lecture might give a slightly different definition.) Things that aren't mechanics are generally considered narrative elements (“this is a war”) or aesthetic elements (the map specifically being a world map, not just a plain diagram), since those aren't distinct things the game does.
Where it gets really broad is that you can look at mechanics from a big picture perspective:
Or from a more zoomed-in perspective:
- Picking units to move is a mechanic
- Moving those units is a mechanic
- Designating combatants is a mechanic
- Rolling for combat a mechanic
- Choosing casualties is a mechanic
- The continue/retreat choice is a mechanic
- Retreat is a mechanic
And both of these lists are more or less true: these are all mechanics that can be considered sub-mechanics of the combat mechanic.
Risk does a lot of things. And all of those things it does almost certainly contain more specific, smaller units of things it does. This means the cutoff point for what you consider a mechanic and how zoomed in you want to go is kind of arbitrary and depends on what you personally are trying to get out of this analysis.
The mechanics you've identified are all accurate so far, but far from the end of the list. The risk cards are a mechanic. They also consist of further mechanics: drawing a risk card is a mechanic, playing it is a mechanic, the things the individual risk cards can do are mechanics. They might engage with other mechanics: it's conceivable a risk card can cause combat to happen, for example.
I suspect you're having some trouble because you're not sure what is and isn't a mechanic or how far to zoom in and break things down, and well, the mechanics are almost all of it. Almost every single thing that makes up the game is a mechanic somehow, aside from the aesthetics and narrative. Your homework assignment needs you to consider this, and you can drill down as far as you feel you need to meet what it's asking of you.