What action must be taken to test the reading lights on the aircraft?

To test the reading lights on the aircraft, the appropriate action is to press and hold the TEST button. This procedure is standardized as it allows for a comprehensive check of the reading lights without altering any other systems on the aircraft. The TEST function is designed specifically for this purpose, ensuring that crew members can verify that the lights are operational and functioning properly.

The other choices involve actions that do not specifically cater to testing the reading lights. For example, turning the lights off then back on may not provide a thorough check of their functionality, as it only cycles the power. Activating the main power switch affects multiple systems and is not directly related to testing the individual unit. Similarly, pressing and holding the emergency button is intended for safety protocols rather than functionality tests for reading lights. Therefore, choosing to press and hold the TEST button is the most effective means of verifying that the reading lights are working correctly.