Never transmit messages within _____ of UXO when using a vehicle mounted radio.

The correct choice is based on safety protocols established for operating near unexploded ordnance (UXO). The recommended distance of 100 feet for transmitting messages while using a vehicle-mounted radio is grounded in the understanding that radio transmissions can potentially trigger detonations or other hazardous reactions from unstable munitions. This distance creates a safety buffer to minimize the risk of accidentally activating UXO, thereby protecting personnel and equipment.

In military operations, ensuring safe communication while maintaining operational effectiveness is crucial. Hence, adhering to this 100-foot rule enhances situational awareness and ensures that all personnel can operate safely in areas that may have unexploded munitions.