Which type of light signals that a vessel is being towed?

The towing light is specifically designed to signal that a vessel is under tow. According to the COLREGs (International Regulations for Preventing Collisions at Sea), the towing light is typically an all-round white light that is displayed above the stern of the towing vessel and above the towed vessel. This light indicates to other vessels that the towing operation is in progress, which is critical for ensuring safe navigation and avoiding collisions. It helps other mariners to understand the vessel's status and adjust their navigation accordingly, especially at night or in poor visibility.

The sidelights are used to indicate the red and green sidelights of a vessel, which show the port and starboard sides, respectively, and do not specifically convey information about a towing operation. The masthead light is used to indicate the forward position of a vessel and does not represent a towing situation either. Flashing lights are often used for specific signals or for vessels engaged in certain activities, but they do not specifically denote that a vessel is being towed. Therefore, the towing light is the correct answer as it uniquely identifies a vessel that is being towed.