The correct answer is A) A vessel towing unable to deviate from her course. A vessel "restricted in her ability to maneuver" refers to a vessel that, due to the nature of her work, is unable to freely maneuver or change course. This is the case for a vessel engaged in towing, as the towing operation restricts the vessel's ability to maneuver as she must maintain her course and heading to safely tow the other vessel. The other options are incorrect because they do not accurately describe a vessel "restricted in her ability to maneuver." A vessel constrained by her draft (B) or not under command (C) may have limited maneuverability, but they are not specifically "restricted in her ability to maneuver" as defined in the navigation rules. A vessel underway in fog (D) is not inherently restricted in her ability to maneuver.