Which of the following types of clothing should not be worn when there is a risk of electrical arc or flash?

The reason nylon should not be worn when there is a risk of electrical arc or flash is that it is a synthetic material that can melt and stick to the skin when exposed to high temperatures or an electrical arc. This melting can cause severe burns upon contact with skin, increasing the risk of injury in hazardous conditions.

In contrast, materials such as cotton and denim are considered safer options for electrical work because they are more flame-resistant and do not have the same properties that lead to melting. Leather is also sturdy and provides a degree of protection against electrical hazards while being durable for work environments. Thus, selecting appropriate clothing is crucial in minimizing injury risks in scenarios involving electrical arcs or flashes.