Which part of the firearm loads, fires, and ejects shells or cartridges?

The action of a firearm is the crucial component that is responsible for loading, firing, and ejecting shells or cartridges. It encompasses various mechanisms that facilitate the operation of the firearm, allowing it to cycle through these processes seamlessly.

The barrel is primarily where the projectile travels after being fired and affects accuracy, while the stock provides stability and support but does not engage in the firing process. The trigger is the control used to release the firing mechanism, but it does not perform the functions of loading or ejecting cartridges. Thus, the action is the correct choice as it encompasses all the mechanisms involved in the operation of the firearm.