When this creature enters, you may sacrifice a creature or land. If you do, put two +1/+1 counters on this creature.
Players can respond to the enters-the-battlefield ability (notably, while Harvester Troll is still 2/3), but once that ability starts resolving, it’s too late for any player to respond. You don’t choose which creature or land you’re sacrificing, if any, until the ability resolves.
