What is the role of a "sentinel"?

The term "sentinel" is defined as a guard or watchman, specifically someone who stands watch and keeps a lookout for potential dangers or threats. This role encompasses vigilance and the responsibility of observing to ensure safety and security, often in military or strategic contexts. The essence of being a sentinel lies in the active monitoring of surroundings and alerting others of any abnormalities or risks.

The other roles mentioned in the choices do not align with this definition. A leader in negotiations focuses on mediating discussions rather than guarding. A historian's duties involve the study and documentation of past events, while an entertainer's purpose is to captivate audiences, none of which entail the protective or vigilant duties associated with being a sentinel.