What signals that a vessel is constrained by her draft?

Answer: A — Three red all-around lights vertically stacked

The correct answer indicates that a vessel constrained by her draft will display three red all-around lights vertically stacked. This specific signaling is important because it communicates to other vessels that the ship is unable to deviate from its course due to the depth of water it requires to navigate safely. The presence of three red lights stacked vertically clearly identifies the vessel's condition to others on the water, helping to prevent accidents and ensuring that other vessels can take the necessary precautions when navigating near her. This is part of the vessel's responsibility to signal its limitations to ensure safety on the water. Other options may correspond to different vessel types or situations, but they do not apply to a vessel constrained by its draft. For example, two masthead lights are used for vessels over a certain length indicating a different operational status, while a single white light and a green light on the starboard side serve distinct purposes that do not communicate draft constraints.