Which type of wiring is not suitable for installations that could be directly exposed to moisture?

Type NM cable, or Non-Metallic Sheathed cable, is not suitable for installations that could be directly exposed to moisture because it is designed for dry locations. The insulation of NM cable does not provide adequate moisture protection, which can lead to potential degradation of the cable and pose safety hazards.

In contrast, Type UF (Underground Feeder) cable is specifically engineered for use in wet locations; it is resistant to moisture and can be buried underground without deterioration. Type THHN (Thermoplastic High Heat-resistant Nylon-coated) wire may also be used in some wet locations when correctly installed, especially if protected by conduit. Type FPLP (Fire Power Limited Plenum) cable is used in specific environments, such as air-handling spaces, and generally has a moisture-resistant jacket appropriate for such conditions.

Thus, Type NM cable's limitations in moisture exposure make it unsuitable for direct installation in such environments, which underscores why it is the correct answer.