No wiring of any type shall be installed in ducts used to transport _____.

The correct answer is associated with regulations pertaining to safety and code compliance in the installation of electrical wiring. The NEC emphasizes that wiring must not be installed in ducts that transport flammable vapors. This is primarily due to the high risk of fire and explosion that could arise from the presence of flammable materials, which can ignite if exposed to electrical sparks or overheating.

Flammable vapors are particularly dangerous because they can create an explosive environment if the concentrations become high enough, especially in confined spaces like ducts. Hence, the NEC prohibits any wiring installations in these ducts to prevent potential ignition sources from being present in areas where flammable vapors circulate.

In contrast, while hazardous materials and airtight substances may also pose risks, they do not specifically address the particular concern regarding flammability and ignition sources that flammable vapors do. Electrical equipment can be used in various applications but does not relate specifically to the transport of hazardous materials or vapors in ducts, making the choice of flammable vapors the most relevant and critical concern in this context.