Which of the following may be a source of ignition when an ambulance is parked off the roadway in a field of dried grass?

The catalytic converter is a component of a vehicle's exhaust system that helps reduce harmful emissions. It operates at high temperatures, especially after the engine has been running. When an ambulance is parked in a field of dried grass, the heat generated by the catalytic converter can ignite the grass, posing a significant fire hazard.

Unlike the mobile radio or battery, which do not create the high levels of heat necessary to ignite grass under normal circumstances, the catalytic converter can reach temperatures well above the ignition point of dried vegetation. Leaking coolant, while it may indicate a malfunctioning engine, does not typically present a fire risk in the same manner. Therefore, the catalytic converter is indeed the most plausible source of ignition in this scenario, leading to the potential for a fire in the surrounding dry grass.