What must be activated to use the toilet on the aircraft?

To use the toilet on the aircraft, it is essential to activate the FURN switch. This switch controls the overall functioning of the aircraft's facilities, including the toilet system, ensuring that all necessary utilities are available for operation. Activating the FURN switch allows for the proper functioning of electrical components, heating elements, and plumbing systems associated with the restroom facilities.

While the other options may play a role in the toilet operation, they are not as critical as the FURN switch. For instance, the water supply valve may need to be open for the toilet to function, but it generally is not controlled directly by the user in the same way the FURN switch is. The power indicator light serves as a status indicator, but it does not enable the system. The emergency flush mechanism, while important in specific situations, is not the primary means for regular operation of the toilet. Thus, the FURN switch is the key component that must be activated to ensure that the toilet facilities are operational and ready for use.