Which unit has the primary mission of constructing advance base facilities?

The primary mission of constructing advance base facilities is specifically aligned with the responsibilities of the Naval Marine Construction Battalions (NMCB). These battalions are designed to provide construction support in various environments, particularly in forward operating areas and during amphibious operations. Their skills include building roads, airfields, and other crucial infrastructure that enables military operations to be carried out effectively from a distance.

On the other hand, other units have distinct but complementary roles. Naval Engineering Support Units (ESU) focus on providing engineering support, planning, and logistics, rather than the direct construction of facilities. Naval Construction Groups (NCG) serve a broader command and control function, overseeing multiple construction battalions and ensuring they are coordinated and effective in their missions. Meanwhile, Naval Facilities Command (NAVFAC) is primarily responsible for managing naval shore facilities and infrastructure but does not engage directly in the hands-on construction of advance bases.

Thus, the role of the NMCB is specifically tuned to the unique needs of constructing essential facilities in tactical environments, differentiating it clearly as the correct answer.