What condition is necessary for external aircraft lighting switches to function?

For external aircraft lighting switches to function, the battery switch must be in the ON position. This is because the external lighting systems, such as landing lights, navigation lights, and taxi lights, require electrical power to operate effectively. The battery provides that essential power when the aircraft is on the ground and not connected to an external power source.

While options related to the generator switch and the avionics master are important for other systems, they do not specifically pertain to the operation of external lights. The generator can supply power when the aircraft is in the air or running, but the immediate requirement for the lights is the battery being on. The condition about the landing gear being locked is unrelated to the lighting functions, as the lights can operate independently of the gear position unless specified by specific systems, such as the landing lights being retracted when not in use or during gear operation.