Which of the following conductor insulations provides the best resistance to heat?

THHN (Thermoplastic High Heat-resistant Nylon-coated) insulation is designed to withstand high temperatures, making it suitable for various applications, especially in environments where heat resistance is vital. It can handle temperatures up to 90°C in dry locations and up to 75°C in wet locations. This characteristic makes THHN an ideal choice for use in conduits and other applications requiring durability under thermal stress.

In contrast, NM (Non-Metallic Sheathed Cable) typically has less heat resistance as it is mostly designed for residential wiring in low-temperature settings, making it unsuitable for high-heat environments. MTW (Machine Tool Wire) has some heat resistance but is primarily designed for use in machinery and has limitations compared to THHN. UF (Underground Feeder) cable is built for underground installations and is also less suitable for extreme heat situations compared to THHN since it's generally used in situations where moisture protection is more critical than heat resistance.

Thus, THHN offers the best performance when it comes to resisting heat due to its specialized design and materials, making it the most reliable choice in applications where heat is a significant concern.