You choose which player or planeswalker Impetuous Sunchaser attacks.

Impetuous Sunchaser attacks only if it's able to do so as the declare attackers step begins. If, at that time, it's tapped or affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost. If that cost isn't paid, Impetuous Sunchaser won't attack.