The answer to your question would be that the best combination of the two sentences would be the following one: During the American Revolution, the war for political independence from Britain, George Washington proved to be a gifted military strategist. That is, your answer would be D.
This is the best combination of the two sentences because it includes the information regarding the war as an apposition, which is a relationship between two or more words or phrases in which the two units are grammatically parallel and have the same referent. The American Revolution is also defined as the war for political independence from Britain, so the information in the second sentence can clearly be included in an apposition. Then, the information regarding George Washington is part of the main sentence because it is brand new information.