What light indicates that a vessel is being towed along with a composite unit?

The correct light that indicates a vessel being towed along with a composite unit is the all-round white light. In maritime navigation, a composite unit refers to a vessel that includes a towing vessel and the towed vessel as a single entity for the purposes of visibility and navigation.

An all-round white light is employed to provide clear visibility of the unit at night and in poor weather, signifying a vessel that is being towed when it is part of a composite unit. This light ensures that other vessels can identify the presence of a towed vessel and understand that it is part of a larger operational unit.

Sidelights only would be used for a single vessel, not for signaling a composite unit's towed status, while a masthead light is primarily for the main vessel or for power-driven vessels operating under certain conditions. A blue light is not commonly used in this context, as it typically serves a different purpose or indicates specific requirements unrelated to towing operations.