Final answer:
The maximum size for threaded fittings in gas piping can exceed 2.5 inches and often goes up to 4 inches or larger, depending on system requirements. The correct size should be determined based on local codes and the National Fuel Gas Code.
Step-by-step explanation:
The maximum size for threaded fittings in gas piping systems isn't provided in the options listed (a) 1 inch, (b) 1.5 inches, (c) 2 inches, or (d) 2.5 inches. This specification can vary based on local codes, regulations, and the specific application. It's important to consult current building codes or industry standards such as the National Fuel Gas Code for the correct information. In general practice, gas piping systems may use threaded fittings that are larger than those options, with common sizes often going up to 4-inch pipe or larger, depending on the demands of the system. As for the provided physics question related to gauge pressure, it is not directly related to the topic of gas piping sizes and is a separate issue.