What is the function of the trigger in the M9 service pistol?

The function of the trigger in the M9 service pistol is to release the hammer. When the trigger is pulled, it engages a mechanism that allows the hammer to move forward, striking the firing pin, which in turn ignites the round in the chamber. This action is critical for firing the pistol. The design ensures that the user can effectively and safely initiate the firing sequence with a single, deliberate action.

The other options represent functions that are not part of the trigger's role. Ejecting spent cartridges is handled by the action of the slide cycling after a round is fired, loading a round is managed by the magazine and slide operations, and securing the grip relates to how the user holds the pistol, rather than a mechanical function of the trigger itself. Understanding these functions highlights the importance of the trigger's role in operation and safety during firearm handling.