Suppose that in response to the trigger the Larva is removed from play (bounced, plowed, sacced, whatever).
When its ability resolves can I choose to sac two lands if I want to (e.g. planning on playing Balance)??
Yes. The ability allows you to choose to either sacrifice two lands or not instead of sacrificing the Cosmic Larva, and it doesn't check to see if the Larva is in play or not. If you sacrifice two lands (and you can only choose this option if you can sacrifice two lands), then you won't have to sacrifice the Larva (even if it isn't in play). By the same token, you aren't forced to sacrifice the lands if someone removes the Larva from play; the ability will simply resolve and be unable to do anything.
