Which of the following conductor types has a flame-retardant, heat-resistant, thermoplastic insulation?

The type of conductor that features flame-retardant, heat-resistant, thermoplastic insulation is THHN. This designation stands for "Thermoplastic High Heat-resistant Nylon-coated" wire. The insulation material is designed to withstand high temperatures while also being resistant to flame, making it suitable for a variety of applications, especially in commercial and industrial settings where heat and fire resistance are critical for safety.

THHN wire is typically used in dry locations and for various electrical installations, where its durability and resistance to physical and environmental stresses can be fully utilized. The heat-resistance property ensures that the wire can operate under higher temperatures without degrading, which is essential for maintaining electrical performance and safety.

The other conductor types listed may have different types of insulation materials that do not emphasize high heat resistance or flame retardancy to the same extent as THHN. For example, UF (Underground Feeder) wire is designed for direct burial applications but does not possess the same level of heat-resistance as THHN. NM (Non-metallic Sheathed Cable) is primarily used for residential wiring and has different insulation characteristics. MC (Metal Clad) cable is built for protection and durability in various environments but may not necessarily feature insulation that is both flame-retardant and heat