What type of clothing is typically worn under flight suits for thermal protection?

The correct answer is fire-resistant clothing because it is specifically designed to provide thermal protection in high-heat environments, which is critical for aircrew members in the event of a fire or extreme temperatures. Fire-resistant materials are engineered to self-extinguish and reduce the likelihood of burns, offering a critical layer of safety when worn under flight suits, which are often made from materials that do not inherently provide protection against flames.

While other types of clothing like cotton, polyester, or denim may be comfortable for everyday wear, they do not offer the same level of protection. Cotton can catch fire easily and doesn't provide any significant thermal insulation in hazardous situations. Polyester can melt under high temperatures, potentially causing severe burns to the skin. Denim, while durable, is also not fire-resistant and doesn’t meet the safety standards required for aircrew personnel. Therefore, wearing fire-resistant clothing beneath flight suits ensures that aircrew members are better protected during flight operations.