A vessel displaying three red lights in a vertical line is __________.

The correct interpretation of a vessel displaying three red lights in a vertical line is that it indicates the vessel is constrained by her draft. This signal is specifically used to communicate to other vessels that the ship's ability to navigate is significantly limited due to its deep draft, which restricts its movement in certain conditions, such as depth of water.

When a vessel is constrained by its draft, it may not be able to maneuver away from another vessel or obstacle in the water due to its design and loading, making this light configuration an important signal for other mariners to understand. This warning helps to prevent collisions and ensures that other vessels can take appropriate measures to give the constrained vessel enough room to operate safely.

Other options would convey different conditions or situations for the vessel, but only the vertical arrangement of three red lights specifically relates to the constraints imposed by a deep draft.