What document assigns the missions and geographic areas of responsibilities to combatant commands?

The correct answer is the Unified Command Plan. This document is vital as it outlines the various missions and assigns specific geographic areas of responsibility to each combatant command within the U.S. military structure. The Unified Command Plan ensures that there is a clear understanding of operational responsibilities and boundaries among the different commands, which helps in coordinating military efforts across diverse regions.

In contrast, other documents mentioned serve different purposes. The Military Strategy Document often encompasses broader strategic objectives and approaches for the armed forces but does not specifically assign geographic responsibilities. The National Defense Strategy focuses on overarching national defense priorities and guidance, but again does not delineate combatant command missions or areas. Operational Guidelines, while important for directing operations and tactics, do not hold the authority to assign missions or geopolitical responsibilities like the Unified Command Plan does.