There are two classes of vessels that do not have to comply with the rule regarding traffic separation schemes. One of these is a vessel __________.

The choice referring to a vessel servicing a navigational aid is correct because vessels performing this function are typically involved in maintenance operations that are essential for safe navigation and may need to deviate from standard traffic separation schemes to carry out their tasks efficiently and safely.

This exemption is key because these vessels often operate in areas that may not be conducive to the adherence of typical maritime traffic rules, especially if they are performing critical repairs or maintenance work that aids in navigation. Such activities are necessary for the overall safety and effectiveness of maritime operations.

Other types of vessels, such as those that are engaged in fishing or carrying hazardous materials, still need to observe the established traffic separation schemes to promote safety and avoid collisions. Similarly, vessels under sail only are required to comply with these rules to ensure they navigate safely in waters shared with other vessel types.