Which of the following materials should never be worn in the presence of potential electric arc or flash?

Nylon should never be worn in the presence of potential electric arc or flash due to its flammability and the fact that it can melt and stick to the skin when exposed to high temperatures or electrical arcs. This property makes it particularly dangerous in situations where an electrical accident could occur, as the melted nylon can cause severe burns and additional injury to the wearer.

In contrast, materials like wool, although not ideal, tend to be more flame-resistant and do not melt, providing somewhat better protection. Silk, while also not the best choice due to its potential to ignite, does not melt onto the skin as nylon does. Canvas, often used in work clothing, can provide some level of protection as well. Therefore, the main reason for avoiding nylon in electrical work is its high risk of melting and adherence to skin during electrical incidents, making it the least safe choice in such environments.