In late summer of 1781, washington’s troops trapped cornwallis’s army at. a) princeton. b) yorktown. c) boston harbor. d) philadelphia.

B. Yorktown
which is caused by 1) poor communications in the British establishment, and (2) French naval superiority over the Chesapeake Bay. 
There was a battle in Princeton and a war in Boston Harbor. 
Cornwallis joined a campaign in Philadelphia.
So, the answer is Yorktown. hope it helps