Which of the following is NOT an appurtenance?

An appurtenance refers to an accessory or supplementary component associated with a larger apparatus or system, often contributing to its proper function. In the context of a system such as plumbing or gas lines, appurtenances include devices or fittings that help manage the flow, safety, or operation of the system.

The mounting bracket typically serves as a support mechanism for securing components rather than directly influencing the operational function of the system, which differentiates it from the other options listed. Pressure relief valves, regulators, and connector fittings are all essential to the regulation, safety, and operation of systems, effectively making them appurtenances. The mounting bracket, on the other hand, does not fulfill a functional purpose within the system itself, placing it outside the category of appurtenances.