When is the mode of a triggered ability selected?

The selected answer, which states that the mode of a triggered ability is chosen when it is put on the stack, is correct because this is the specific moment in the process of resolving abilities where the controller of the ability has the opportunity to choose how the ability will function.

When a triggered ability occurs, it goes through several steps before it takes effect. Initially, the ability triggers and is then placed on the stack. At this point, the player who controls it can choose the mode if the ability allows for multiple modes. This typically happens with abilities that specify various options, like choosing a creature or an effect.

Once the ability is on the stack, it becomes a part of the game state waiting to resolve. At this stage, the decision on which mode to use is made, and the selected mode will dictate how the ability will resolve once it gets its chance to do so.

As for other options, they do not correctly align with the rules governing triggered abilities. The activation of an ability does not occur at the point where the mode is selected, and the resolution of the ability comes after mode selection has already been made. Starting the game does not pertain to triggered abilities at all since they only come into play based on certain events