Final answer:
The M4 Handguard in the M4 Rail Adapter System may be removed for PMCS.
Step-by-step explanation:
In the M4 Rail Adapter System, the part that may be removed to perform PMCS (Preventive Maintenance Checks and Services) is the M4 Handguard. The M4 Handguard is a component of the M4 Rail Adapter System that provides a platform for attaching accessories such as optics, lasers, and grips. By removing the M4 Handguard, the user can access the internal components of the rifle for maintenance and inspection.