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

The action of a firearm is the mechanism that is responsible for loading, firing, and ejecting shells or cartridges. It includes all the moving parts that facilitate these functions, such as the bolt, slide, or lever, depending on the type of firearm. When the shooter pulls the trigger, it signals the action to engage in the sequence of operations that allows the firearm to function.

The stock is primarily the part of the firearm that provides a resting place for the shooter's shoulder and helps stabilize the firearm during shooting. The barrel is where the bullet travels through after it is fired, influencing accuracy and range. The trigger is the component that the shooter pulls to initiate the firing process, but it does not perform the loading or ejecting functions itself. Therefore, the action is the correct answer as it encompasses the entire operation of loading, firing, and ejecting cartridges.