A vessel, which does not normally engage in towing operations, is towing a vessel in distress. She _____________.

The most appropriate choice is that the vessel need not show the lights for a vessel engaged in towing if it is impractical to do so. This is because the regulation allows for flexibility in situations where a vessel, which is not typically in the towing business, becomes involved in a towing operation under emergency conditions, such as towing a vessel in distress. In such scenarios, if it is not feasible to display the required towing lights due to the circumstances or limitations of the vessel, the operator is excused from this requirement.

This option reflects a pragmatic approach in navigation rules, acknowledging that the primary goal is safety and practicality on the water. Vessels not dedicated to towing may not have the appropriate systems in place, and the law recognizes that adhering to every detail of the lighting rules may not always be possible in emergencies.

The other choices present scenarios that do not fit the context of a vessel that is not normally engaged in towing operations. For instance, showing the lights for a vessel not under command or needing a yellow light would apply under different circumstances and regulations that are not relevant when a vessel temporarily assists another in distress.