Descending boats, while awaiting their turn to enter a lock, shall NOT block traffic from the lock. They shall be above the lock by at LEAST __________.

Correct Answer: Option D — 400 feet

Navigation regulations for lock operations require vessels to maintain a safe distance to ensure traffic flow and prevent accidents. Descending vessels must remain at least 400 feet above the lock while awaiting their turn, preventing congestion and allowing sufficient room for vessels exiting the lock chamber.