Situation: My opponent has a Zombie Army with 3 +1/+1 counters, and also a another Zombie Army (say for example, Amoeboid Changeling). They cast a card with Amass (for example: Relentless Advance). I have a Jaya's Greeting in hand.
Question: Can I choose to respond to the Relentless Advance by destroying whichever creature will get the tokens? Or does my opponent choose after it resolves?
Amass doesn't target, so the creature will be chosen upon resolution, by which time it is too late to destroy it with Jaya's Greeting; the creature will receive the +1/+1 tokens before you get priority to cast a spell.
As the other answer, Amass doesn't target. You can preemptively kill the changeling or the token before the resolution. Remember that if you choose to destroy the token, then your opponent can only place the counters in the changeling(and vice-versa), if you destroy both creatures, he would then create a zombie army token as per Amass's text.
Amass N (Put N +1/+1 counters on an Army you control. If you don't control one, create a 0/0 black Zombie Army creature token
Look at the reminder text, it says put the counters on an army you control, not on target army you control. If it had the word "target" you would need to target on cast, as it doesn't, it amasses on resolution of the spell.
