Which vehicles are not required to display mast lights?

The correct answer highlights specific vessel categories that are exempt from displaying mast lights according to maritime regulations. Vessels classified as "NUC" (Not Under Command), "sail," "fishing," and "towed" have their own distinct lighting requirements based on their operational status or type.

For instance, the Not Under Command vessels are not able to maneuver as required, thus have unique signal lights to indicate their condition, which may replace or supplement the traditional mast lights. Similarly, sailing vessels often have specific lighting configurations that differ from motorized vessels, while fishing boats may use alternative lights that signify their activities. Towed vessels, while typically concerning tow lights, can also fall under different regulations depending on their situation.

This flexibility in lighting requirements is designed to enhance safety and communication on the water, ensuring that vessels in particular operational states are clearly marked to other mariners. The other options present broader categories of vessels that either mandate mast lights for all operating scenarios or are less specific, which is not aligned with the exceptions provided to certain types of vessels based on their functionality or status on the water.