Final answer:
The diameter of a pressure gauge for a leak test on gas piping is based on readability and accuracy, without specific standards, all provided diameters could be correct.
Step-by-step explanation:
The minimum diameter of a pressure gauge needed to perform a leak test on gas piping is not provided in the question. However, when working with pressure gauges for such tests, the diameter of a pressure gauge typically relates to the readability and accuracy required for the measurement, rather than being a determinant factor in the capability to test for leaks. A standard diameter for many pressure gauges is around 2 inches, but this can be larger for gauges that need to be readable from a distance or where greater detail is needed in the reading. For this reason, without specific codes or standards referenced, any of the diameters provided in the answer options A. 2", B. 2 - 3/4", C. 3", or D. 3 - 1/2" could potentially be used, depending on the precise requirements of the leak test being performed.