The Marine Corps calls its senior noncommissioned officers (NCOs) ______________.

The term "staff NCOs" refers to senior noncommissioned officers within the United States Marine Corps. In this context, staff NCOs include ranks such as gunnery sergeants, master sergeants, and first sergeants, who play a crucial role in leadership, training, and mentoring junior Marines. They are responsible not only for carrying out duties but also for instilling discipline and ensuring the effectiveness of their units. This designation emphasizes their experience and authority within the enlisted rank structure, distinguishing them from lower-ranking noncommissioned officers.

The other options represent different roles or branches within the military context. "First sergeants" is a specific rank within the staff NCO category, while "chief petty officers" is a designation used in the Navy, not the Marine Corps. "Master sergeants" is also a rank among staff NCOs but does not capture the broader category that includes all senior NCOs.