Flameblast Dragon — ruling 1

You choose the target when the ability triggers. When the ability resolves, you choose a value for X and decide whether to pay {X}{R}. If you do decide to pay {X}{R}, it's too late for any player to respond since the ability is already in the midst of resolving.

Flameblast Dragon : règles, fonctionnement et interactions (MTG)

Flameblast Dragon 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

Flying Whenever this creature attacks, you may pay . If you do, it deals X damage to any target.

How it works : Flameblast Dragon

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2008· Wizards of the Coast
You choose the target when the ability triggers. When the ability resolves, you choose a value for X and decide whether to pay . If you do decide to pay , it's too late for any player to respond since the ability is already in the midst of resolving.
No other card is named in this ruling.