What do three red lights in a vertical line indicate?

Three red lights in a vertical line indicate a vessel constrained by her draft. This configuration is part of the navigation lights prescribed by the International Regulations for Preventing Collisions at Sea (COLREGs). When a vessel is constrained by her draft, it means that she is unable to navigate as she normally would due to the depth of water and her size, making it a significant hazard to other vessels.

The display of three red lights signals to other vessels that they should exercise caution and be aware of the limitations that the constrained vessel is experiencing. It's essential for mariners to recognize this signaling to ensure safe navigation in close proximity to such vessels. The appropriate understanding of these signals is crucial for maintaining safety and preventing collisions on the water.

In contrast, the other options refer to different scenarios and would have distinct signaling configurations. A vessel at anchor would typically display a single white light, a vessel not under command would display two red lights, and a vessel towing another would show a different arrangement depending on the number of vessels being towed and other factors. Each signal serves a specific purpose, ensuring that the intentions and limitations of vessels in various situations are clearly communicated to others on the water.