Which of the following describes a reserve?

A reserve refers to a crewmember available on a 24-hour call. This status signifies that while they may not be actively scheduled for a specific flight, they are on standby and can be contacted to cover flights if needed. This arrangement helps airlines maintain flexibility in staffing, ensuring there are enough crew members available to address any unforeseen scheduling needs or operational disruptions.

The other choices do not align with the definition of reserve. For instance, being a crewmember not assigned to any flights during a shift describes a different status, and a passenger with a standby ticket is related to passenger boarding procedures rather than crew assignments. Similarly, a flight delayed overnight does not pertain to a crewmember's status or availability. Thus, being on a 24-hour call accurately captures the essence of what being a reserve means in the context of airline operations.