The correct answer is A. Warships were patrolling the river and harbor.
At the Siege of Yorktown in 1781, the British forces under General Cornwallis were effectively trapped. The French fleet, along with the American and French ground forces, blockaded the Chesapeake Bay and prevented any British ships from coming in or going out, which meant Cornwallis could not escape by sea. This blockade was a crucial factor in the British inability to retreat successfully from the siege.