Rule 602.3 · Activating Activated Abilities
Some abilities specify that one of their controller’s opponents does something the controller would normally do while it’s being activated, such as choose a mode or choose targets. In these cases, the opponent does so when the ability’s controller normally would do so.
Subrules
If there is more than one opponent who could make such a choice, the ability’s controller decides which of those opponents will make the choice.