Final answer:
The first cartridge loaded into a magazine presses against the follower, a part of the magazine that moves under spring tension to feed cartridges into the chamber.
Step-by-step explanation:
The first cartridge loaded into a magazine presses against the D. Follower.
The follower is a part of the magazine that moves under spring tension to feed cartridges into the chamber. When the magazine spring is uncompressed, as shown in FIGURE 16.7(a), there is no external pressure applied. Upon loading a cartridge and pressing it down against the follower, the spring beneath it compresses. This action is further depicted in the scenario where 'the spring has been compressed a distance', indicating that a cartridge has pushed down on the follower, which in turn compresses the spring. The follower is what directly contacts the cartridges and ensures that they are loaded into the firearm correctly and reliably.