Which material is prohibited for gas fittings when installed outdoors or underground?

The correct choice is aluminum alloy because it is not suitable for use in gas fittings when installed outdoors or underground due to its susceptibility to corrosion. When aluminum is exposed to certain environmental conditions, particularly moisture and various soil types, it can corrode over time. This corrosion can lead to the degradation of the piping system, creating potential leaks or failures, which poses significant safety risks.

On the other hand, steel, when properly coated or galvanized, is often used for gas piping in outdoor conditions because it provides better durability and resistance to external elements. Plastic piping, while not used for every application, is specifically designed for underground installations in certain conditions due to its resistance to corrosion and lightweight properties. Copper is also frequently used for gas fittings both above ground and underground but is not typically as problematic in outdoor applications as aluminum alloys.