Which document provides guidance for body composition in the USMC?

The correct answer is indeed MCO 6110.3A. This specific Marine Corps Order outlines the policies and procedures regarding body composition for all members of the United States Marine Corps (USMC). It provides detailed guidance on how body composition is assessed, the standards that must be met, and the consequences for failing to meet these standards. The MCO serves as a critical resource for ensuring that Marines maintain a healthy and fit physique, which is vital for operational readiness and overall health.

While other documents mentioned may contain relevant information regarding fitness or training, they do not specifically focus on body composition standards for the USMC in the same way that MCO 6110.3A does. The comprehensive nature of MCO 6110.3A makes it an authoritative source for anyone in the Marine Corps looking to understand and adhere to body composition guidelines.