A credentialed Master of Towing Vessels may NOT serve as the Captain of which towing vessel?

Correct Answer: Option D — 375 GRT towing vessel operating between Texas and Puerto Rico

Under USCG credentialing regulations, a standard Master of Towing Vessels endorsement restricts the holder from acting as master on towing vessels exceeding 200 gross tons or operating on unauthorized ocean routes without specific ocean or tonnage upgrades.