Can a player concede in response to a triggered ability to prevent it from resolving?

A player may concede the game at any time, even during another player's turn or while abilities are on the stack (CR 104.3a). Conceding is not a game action that uses the stack — it takes effect immediately and unconditionally.

However, conceding does not automatically counter or remove triggered abilities that are already on the stack. Once an ability is on the stack, it exists as an independent object. The player leaving the game does not cause stack objects to disappear on their own (CR 800.4a). Any triggered abilities already on the stack will continue to be processed normally.

What conceding does affect: if a triggered ability requires its controller (the conceding player) to make choices or perform actions upon resolution, and that player has left the game, the remaining players may be unable to fulfill those requirements — but the ability still resolves and does as much as it can (CR 608.2b). Permanents the conceding player controlled are removed from the game, which can cause abilities to fizzle if they target those permanents (CR 800.4a).

Concrete example: Your opponent controls a Primeval Titan that attacks, triggering its 'search for two Forests' ability. You concede in response. The ability is already on the stack — but since your opponent controls it, it still resolves and they still get their lands. Conceding did not stop it.

In short: conceding cannot be used as a trick to 'counter' a triggered ability that is already on the stack. If the goal is to prevent an ability from resolving, conceding is generally not an effective tool for that purpose.