Cast iron coupling for joining hub less cast iron pipe shall consist of a ______ gasket.

The correct choice is neoprene as the gasket material for joining hubless cast iron pipe. Neoprene is a type of synthetic rubber that exhibits excellent resistance to harsh environmental conditions, temperature fluctuations, and chemical exposure. This quality makes it particularly suitable for plumbing applications, where durability and resistance to wear are essential.

In the context of hubless cast iron pipes, a neoprene gasket provides a secure and flexible seal that helps prevent leaks while accommodating slight movements or expansions that may occur in the pipe system. Neoprene also offers good compression recovery, which ensures that the gasket maintains its sealing capability over time, even under varying pressure conditions.

While rubber and silicone might initially seem appropriate due to their sealing properties, they do not offer the same level of resistance and durability that neoprene does in plumbing systems. Plastic, on the other hand, is not used for gaskets in this application because it lacks the necessary flexibility and sealing capability required for a reliable joint in a cast iron system.