While on a VFR cross country, what frequency should be used for emergency communication if not in contact with ATC?

Using 121.5 MHz for emergency communication is essential because that frequency is designated as the international distress frequency for aircraft. It is monitored by Air Traffic Control (ATC) and other aircraft, and is intended for emergency situations where a pilot needs immediate assistance. In cases where a pilot is not in contact with ATC, using this frequency ensures that any nearby aircraft or relevant authorities can hear the distress calls and respond accordingly.

Other frequencies, such as 122.75 MHz, are common frequencies used for non-towered airport communications or for general aviation traffic, but they are not designated for emergencies. Similarly, 123.5 MHz is also a common air-to-air frequency and not specifically set aside for emergency situations. Finally, while 120.0 MHz may be used for various purposes, it is also not the recognized emergency frequency. Therefore, 121.5 MHz is the appropriate choice for reporting emergencies when ATC communication is not established.