What is referred to as 'stacked feed' in a firearm?

Stacked feed in a firearm refers to a configuration where rounds are aligned in a magazine in a vertical stack, allowing for the feeding of ammunition into the chamber efficiently. The concept of stacked feed becomes relevant especially when discussing semi-automatic and automatic firearms, where a round must be fed from the magazine into the chamber to be fired.

Option A describes a situation in which a round is indeed in the chamber and the action is closed, signifying that the firearm is ready to fire. In this context, 'stacked feed' emphasizes that the magazine has fed the round into the chamber properly, which is crucial for the firearm to function smoothly and reliably.

The other options, while related to ammunition and its management, do not accurately describe the term 'stacked feed.' For instance, storing ammunition safely or organizing it by caliber focuses on storage practices rather than the feeding mechanism in a firearm. A fully loaded magazine pertains to the capacity of the magazine without indicating the mechanism of feeding the round into the chamber. Thus, the understanding of stacked feed ties closely with the operation and readiness of the firearm.