What prefix is used before the hull numbers for USNS ships?

The correct choice is based on the standard identification of United States Navy Ships, specifically the designation of Military Sealift Command ships, which is indicated by the prefix "T." This prefix stands for "transport," and it is used to identify auxiliary ships and service support vessels that are not part of the combat fleet but are still critical to naval operations.

USNS ships (United States Naval Ships) are specifically government-owned and operated by civilian mariners. The prefix helps distinguish these ships from the operational Navy vessels, which typically do not use the "T" prefix and instead might have other designations according to their mission and function.

In contrast, the prefixes used for other types of naval vessels—such as submarines or combat ships—do not include "T" and instead may use letters such as "P" for patrol boats, "R" for repair ships, or "M" for maritime vessels. Hence, "T" is distinctively associated with the category of logistics and support vessels within the US Navy fleet.