What Field Manual (FM) details the use of crew served machine guns?

The correct choice details the use of crew served machine guns is FM 3-22.68. This Field Manual specifically focuses on the Army's operational doctrine concerning machine guns, including their employment, tactics, techniques, and procedures. It provides guidance on the organization and operation of crew served machine guns, which are essential in various combat scenarios. This manual serves as a critical resource for units that rely on these weapons for suppressive fire and defense.

In contrast, other Field Manuals listed focus on different topics. For example, FM 3-21.5 covers the field manual for drill and ceremonies, FM 3-25.26 pertains to map reading and land navigation, and FM 6-22 deals with army leadership and the leadership development process. Each of these FMs has a specific purpose and audience but does not include the detailed information on crew served machine guns provided in FM 3-22.68.