Rule 403.4

If there are not enough options to make legal choices for a Triggered Ability that has been put on the chain, remove it from the Chain now. It ceases to be a Pending Item but never becomes a Finalized Chain Item.

• 403.4.a This is not an Ability being countered.
• 403.4.b If there are legal options to choose, the ability’s controller must choose them. They may not decline this stage of…

Rulings that cite this rule

• What about triggered abilities with multiple targets, like Sona, Harmonious?
• Can Janna’s play effect be used to heal my units without an enemy unit present?

Official Core Rules