Rule 371.2

If the Replacement Effect says a player “may” apply the Replacement Effect, the player has the choice of whether or not to apply it.

• 371.2.a When an event the Replacement Effect could apply to occurs, the player who controls the Replacement Effect may choose…
• 371.2.b If they do not, it has not been applied this turn. Example: Zilean, Time mage reads “Once each turn, if you would play…

Official Core Rules