Spell Rupture — ruling 1

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.

Spell Rupture : règles, fonctionnement et interactions (MTG)

Spell Rupture is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Counter target spell unless its controller pays , where X is the greatest power among creatures you control.

How it works : Spell Rupture

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2013· Wizards of the Coast
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 (simply by indicating they wish to do so). That player can also choose to not pay and the spell will be countered.
No other card is named in this ruling.