603.5  Some triggered abilities’ effects are optional (they contain "may," as in "At the beginning of your upkeep, you may draw a card"). These abilities go on the stack when they trigger, regardless of whether their controller intends to exercise the ability’s option or not. The choice is made when the ability resolves. Likewise, triggered abilities that have an effect "unless" something is true or a player chooses to do something will go on the stack normally; the "unless" part of the ability is dealt with when the ability resolves.

114.1d  A triggered ability is targeted if it identifies something it will affect by using the phrase "target [something]," where the "something" is a phrase that describes an object, player, or zone. The target(s) are chosen as the ability is put on the stack; see rule 603.3d.