Which of the following frequencies is uniquely associated with emergency locator transmitters (ELTs)?

The frequency of 121.5 MHz is specifically designated for emergency locator transmitters (ELTs). This frequency is crucial because it's a distress frequency recognized internationally for aviation emergencies. When activated, ELTs transmit a signal to alert search and rescue teams of an aircraft's location, playing a vital role in facilitating a quick response during emergencies.

Other frequencies, while they serve important functions in aviation communication, do not have the unique association with ELTs that 121.5 MHz does. For example, 118.5 MHz is commonly used for air traffic control communications, 122.5 MHz is an aviation frequency often used for flight information service broadcasts, and 124.5 MHz is used for civil emergency communications, but none of these frequencies serve as the primary distress signal for ELTs. This specificity of 121.5 MHz as a dedicated emergency frequency highlights its critical importance in aviation safety.