Which step involves using the index finger along the magazine's forward edge to confirm it's filled?

The step that involves using the index finger along the magazine's forward edge to confirm it's filled is referred to as indexing the magazine. This practice is crucial for ensuring that the magazine is fully loaded and ready for use. By sliding the index finger along the forward edge, the user can feel for the cartridges inside the magazine, checking for any potential issues such as a jammed round or that the magazine is not adequately filled. It is a tactile verification that helps ensure the reliability of the firearm when loaded.

The other options pertain to different actions associated with handling a magazine. Seating the magazine refers to the process of inserting the magazine into the firearm until it locks in place, ensuring a proper connection. Engaging the safety involves setting the firearm's safety mechanism to prevent accidental discharge, which is unrelated to confirming the magazine's contents. The loading procedure encompasses the entire process of preparing the firearm for operation, including loading ammunition and securing the magazine, but does not specifically focus on the tactile confirmation method described in the correct response.