Does ward count as a triggered ability that can be responded to?

Ward is a keyword ability defined in CR 702.20. When a permanent with ward becomes the target of a spell or ability an opponent controls, ward triggers automatically. This trigger goes on the stack like any other triggered ability, and all players receive priority and may respond to it before it resolves.

When the ward trigger resolves, the opponent who controls the spell or ability must pay the ward cost (life, mana, discarding a card, etc.). If they don't or can't pay, the spell or ability that targeted the ward permanent is countered by the ward trigger's effect (CR 702.20b).

Because ward is a triggered ability (not a replacement effect or static ability), it can itself be responded to. An opponent could, for example, cast a spell in response to the ward trigger before being forced to pay — though they still must pay when the trigger resolves or lose their spell.

Concrete example: Your opponent controls a creature with Ward {2}. You cast Doom Blade targeting it. Ward triggers and goes on the stack. You both get priority — you could cast an instant in response to ward if you wished. When the ward trigger resolves, you must pay {2} or Doom Blade is countered.

Note that ward only triggers once per targeting event. If the ward trigger is somehow countered (e.g., with Stifle), the cost is never required and the original spell or ability continues to resolve normally (CR 702.20b, 509.2).