The correct answer is "French ships blocked British supply ships, and French troops besieged the city.". Indeed, both the French and American armies were met near New York during the summer of 1781. American troops amounted to 8000 regular soldiers and 3100 militia but they had no warships whatsoever. These was very dangerous since British ships could always bring more troops and supplies with their navy and fire on coastal positions occupied by the rebels. The French brought in about 8200 regular soldiers plus 29 warships. This influx of men, materiel, money and ships was especially invaluable for the Americans as they gave them a clear numeric superiority over the British forces. The French navy blocked the coastline, preventing British General Cornwallis from escaping to fight another day. French and American troops besieged Yorktown together until October 17th, 1781, when Cornwallis and his army surrendered to the Americans.
