What triggers the ability for a double-faced card to transform?

The ability for a double-faced card to transform is indeed triggered by specific conditions that are defined by the card itself or by the game rules. In Magic: The Gathering, double-faced cards contain both a front and a back side, and they can only transform under specific circumstances indicated in their rules text.

Typically, the transformation is triggered by a spell or ability that specifically states when the card can change from its front face to its back face, such as meeting a condition related to the game state, or another effect that allows or requires the transformation to occur. This embedding of transformation conditions is crucial for maintaining the game’s structure and ensuring that transformations happen under precise rules, rather than being left at the whims of players.

The other options do not align with the mechanics set by the game. A player cannot arbitrarily choose to transform a double-faced card; it is dependent on the specified criteria. The end of the game does not trigger transformation but instead concludes all ongoing activities within the game. Lastly, while discarding another card may sometimes be a relevant action in the game, it does not inherently provide a mechanism for transforming a double-faced card unless stated by a card ability or rule.