Which of the listed pipe sizes is not commonly used?

Correct Answer: Option C — 5/8 inch

Standard American pipe sizes follow a specific progression based on nominal pipe size (NPS) standards. While 3/8, 1/2, and 3/4 inch are common industry-standard dimensions for piping and fittings, 5/8 inch is not a recognized standard size in marine piping systems, making it unavailable for routine maintenance or installation.