What is the function of the equipment that emits a distinct audio tone on 121.5 MHz and 243.0 MHz?

The equipment that emits a distinct audio tone on 121.5 MHz and 243.0 MHz is an Emergency Locator Transmitter (ELT). The primary function of an ELT is to transmit a distress signal to aid in the location of an aircraft that has been involved in an accident or is in distress. The frequencies of 121.5 MHz and 243.0 MHz are designated for emergency use, with 121.5 MHz being commonly used for civilian aircraft and 243.0 MHz for military purposes.

When activated, an ELT broadcasts a continuous signal that can be detected by search and rescue satellites, ground stations, or other aircraft in the vicinity. This capability significantly enhances the chances of locating an aircraft quickly and effectively in emergency situations, ultimately contributing to safety in aviation. The other options serve different purposes: antennas transmit and receive signals, transponders identify aircraft to air traffic control, and VORs provide navigational assistance to pilots.