What type of signals should not be used for vehicles carrying explosives?

Vehicles carrying explosives should not use flares or fusees as signals due to the inherent fire hazard they present. Flares and fusees produce an open flame and high heat, which can ignite explosive materials. The primary safety concern is that anything that generates significant heat or fire should be strictly avoided in the vicinity of explosive loads.

In contrast, electric signals, reflective devices, and light beacons do not pose the same risk. Electric signals can be used without creating an open flame, while reflective devices help to increase visibility without any heat generation. Light beacons, often used for warning when vehicles are stationary, also do not create any risk of ignition. Therefore, flares and fusees are highly discouraged for signaling when transporting explosives, making this the key reason for the correct answer.