Which vessel is required to exhibit white masthead lights?

Answer: C — vessel constrained by her draft.

The correct answer involves understanding the navigation lighting requirements as per the International Regulations for Preventing Collisions at Sea (COLREGs). A vessel constrained by her draft is required to exhibit white masthead lights to indicate her status clearly to other vessels. This class of vessel is in a situation where her ability to navigate safely is restricted due to her draft, meaning that she cannot leave the navigable water without risking grounding. The white masthead lights serve to communicate this restriction to other vessels, helping to prevent collisions and ensuring all mariners are aware of the vessel's limitations. In contrast, a sailing vessel under power would typically exhibit different lights depending on the situation (e.g., sails up or motoring). A tugboat pushing a barge has its own set of lighting requirements that would not include just the white masthead lights; tugboats often have additional lights to denote their towing operation. Finally, a fishing vessel operating at night generally displays a combination of red and green lights along with additional signals related to its fishing gear or status.