Fanatic of Xenagos

Players can’t respond to the tribute decision before the creature enters the battlefield. That is, if the opponent doesn’t pay tribute, the triggered ability will trigger before any player has a chance to remove the creature.

The choice of whether to pay tribute is made as the creature with tribute is entering the battlefield. At that point, it’s too late to respond to the creature spell. For example, in a multiplayer game, opponents won’t know whether tribute will be paid or which opponent will be chosen to pay tribute or not when deciding whether to counter the creature spell.