What signals that a vessel is constrained by her draft?

The correct indication that a vessel is constrained by her draft is signaled by three red all-around lights arranged vertically. This specific configuration is part of the visual signals defined in the COLREGs to clearly indicate the status of various types of vessels. A vessel that is constrained by her draft is not able to navigate as freely as other vessels due to the limitations caused by the depth of the water in which she is operating; therefore, the three red lights serve to alert other mariners of this condition.

By utilizing the three red all-around lights, mariners can quickly recognize that the vessel in question has specific navigational limitations and must be given appropriate space to maneuver safely. This signal is crucial in preventing collisions and ensuring safe navigation in crowded waterways.

The other options pertain to different vessel statuses or types of vessels. For example, two masthead lights indicate that a vessel is a power-driven vessel of over 50 meters in length, while one white light on the bow generally signals a sailing vessel, and a green light on the starboard side is part of a vessel's navigation lights indicating its position but does not convey the constraint caused by draft.