A transition coupling may be used to ______________________________________________.

A transition coupling is specifically designed to connect different types of materials or pipes within a plumbing system while maintaining a secure and leak-free joint. In the case of connecting a PVC schedule 40 tub waste and overflow to a metallic waste pipe, a transition coupling is essential because it can accommodate the differences in material properties and diameters between PVC and metal.

PVC and metal pipes expand and contract at different rates due to temperature changes, which could lead to a failure if not correctly managed. A transition coupling allows for flexibility and can compensate for these shifts while providing a strong joint. This is significant in plumbing applications to prevent leaks and ensure the longevity of the connection.

Other options presented do not utilize a transition coupling properly. Transitioning between schedule 40 PVC and schedule 40 ABS typically involves specific types of fittings designed for joining those two similar materials. Inserting a PVC test tee into an existing stack does not require a transition coupling, as it involves the same type of material and utilizes standard connection methods. Therefore, the correct choice highlights the practical application of a transition coupling in a scenario where two different materials meet, emphasizing its role in ensuring integrity and functionality in plumbing systems.