Which of the following refers to the intelligence function in the US Marine Corps?

The intelligence function in the US Marine Corps is referred to as S2. In military terminology, S2 is specifically tasked with overseeing intelligence operations, including gathering, analyzing, and disseminating information that is crucial for mission planning and execution. The role of S2 encompasses the assessment of threats and the provision of intelligence support to command decisions, which is vital for operational success.

The other options represent different functions within the staff structure. S3 typically focuses on operations, S4 deals with logistics, and S5 is often associated with planning and future operations. Each of these roles plays a distinct part in the overall functioning of a unit, but S2 is specifically designated for intelligence-related tasks.