Which of the following roles primarily assists subordinate commanders and communicates with units outside headquarters?

The primary staff plays a crucial role in assisting subordinate commanders by providing them with necessary information and support. Their responsibilities include coordinating and harmonizing efforts among various units and facilitating communication between the command and subordinate elements. This ensures that operations run smoothly and that all units are aligned with the commander's intent and objectives.

The primary staff serves as a central hub for information, enabling effective communication not only within the headquarters but also outside of it, to units that may be engaged in direct operations or affected by the command's decisions. Therefore, their role in maintaining this continuous flow of information and support is vital for mission success.