Which of the following best describes the term "sentry"?

The term "sentry" primarily refers to a soldier assigned to keep guard. This role typically involves monitoring a specific area or location for security purposes, ensuring that no unauthorized personnel enter, and observing for any signs of suspicious activity. A sentry plays a crucial role in maintaining the safety and security of a military installation, base, or during operations, showcasing the importance of vigilance and responsibility in military duties.

The other options describe different roles within a military or security context but do not accurately capture the specific definition of a "sentry." A civilian security officer is often a non-military role focused on private property or personnel. An officer in command refers to someone who holds a leadership position and is not necessarily involved in guarding. A military commander signifies a higher rank and responsibility, overseeing operations or units, rather than the act of guarding in a sentry capacity.