To verify that an aircraft's ELT is not transmitting before shutdown, what frequency should you listen on?

Listening on 121.5 MHz is the correct choice because this frequency is designated as the international emergency frequency for general aviation distress and emergency communications. When an aircraft's Emergency Locator Transmitter (ELT) is activated, it transmits a distress signal on this frequency, allowing search and rescue services to locate an aircraft in distress.

By listening on 121.5 MHz before shutdown, pilots can confirm that the ELT is not inadvertently activated and transmitting, which helps avoid unnecessary search and rescue operations or interference with emergency communications. It is a critical check to ensure that the aircraft is ready for flight without any issues related to the ELT.