Counter target spell unless its controller pays {X}, where X is the greatest power among creatures you control.
The value of X is the greatest power among creatures you control when Spell Rupture resolves. If you control no creatures at that time, X will be 0. The target spell’s controller can choose to pay {0} (simply by indicating they wish to do so). That player can also choose to not pay {0} and the spell will be countered.