Which of the following cargo types does NOT require the use of triangular reflectors instead of flares?

In the context of transportation safety regulations, particularly regarding hazardous materials, certain materials are subject to specific signaling requirements to ensure safety during roadside stops or emergencies. Triangular reflectors are used instead of flares for certain types of cargo due to the potential for ignition and fire hazards that flares could cause.

Cargo types like flammable liquids, explosives, and gases pose significant risks due to their potential to ignite or explode in the presence of open flames or extreme heat, making the use of flares inappropriate for those materials. Instead, triangular reflectors serve as a safe alternative that can alert other road users without posing an additional fire risk.

On the other hand, solid materials are generally considered less hazardous in the context of ignition and do not typically carry the same risk as flammable liquids, explosives, or gases. Therefore, their transport does not necessitate the same precautions regarding emergency signaling. As a result, the use of triangular reflectors instead of flares is not required for solid materials, making it the correct answer to the question.