In astern towing, a tow span, also called the "tow bar" or "towing arch", is used to __________.

Correct Answer: Option B — prevent fouling of the hawser on deck gear located on the stern of the towing vessel

A tow span or towing arch is installed to keep the hawser elevated above the stern. This prevents the line from fouling on deck gear, bitts, or railings during maneuvers, ensuring the hawser maintains a clear, unobstructed lead and reducing the risk of damage to the vessel.