Conductor insulation must be approved for the of use?

The correct answer to this open question is the following.

Conductor insulation must be approved for the operating temperature of use.

When working in electrical environments such as an electricity plant, it is very important to have the proper safety measures because an accident can be fatal. Electrical insulation its important in any device because it limits the flow of electrical current between the conductor to the ground and between those conductors. Otherwise, the risk would be too much. The insulation material could be rubber or plastic.

the answer would be c