Which title represents the second-in-command on an aircraft?

The title that represents the second-in-command on an aircraft is "First Officer." This role is essential in the cockpit hierarchy, as the First Officer works closely with the Captain to ensure the safe operation of the flight. The responsibilities of a First Officer include assisting with flight planning, conducting communication with air traffic control, and taking charge of the aircraft if the Captain is incapacitated.

The other terms listed do not accurately reflect the standard aviation terminology associated with piloting an aircraft. A Flight Manager is typically a position focused on operational aspects rather than being in the cockpit, while a Crew Leader might refer to a head flight attendant or a similar role within the cabin crew. An Associate Pilot, while it may imply a supporting role, is not a formal title recognized in flight operations. Thus, "First Officer" is clearly the correct and established term used across the aviation industry to denote the second-in-command.