At night, three red lights in a vertical line indicate which type of vessel?

Answer: A — vessel constrained by her draft.

The configuration of three red lights arranged in a vertical line specifically signifies a vessel constrained by her draft. This visual signal is used to indicate to other mariners that the vessel cannot navigate as it would normally due to the limitations imposed by her draft. When a vessel is constrained by her draft, it is typically deep-drafted and unable to maneuver freely because of shallow waters or other hindrances. This warning serves to alert nearby vessels to exercise caution when approaching, as the constrained vessel has restricted ability to change course or speed. The other types of vessels indicated by the options provided have distinct lights that differentiate them from vessels constrained by draft. For example, a sailing vessel would show a combination of shapes or different colored lights, while a fishing vessel would typically display a different arrangement such as fishing lights and torches. A power-driven vessel also has its set of lights, commonly a white masthead light and sidelights. Thus, the three red lights in a vertical line are uniquely designated for vessels constrained by their draft in accordance with maritime navigation rules.