The correct answer is A) to stay informed about the military and its supplies.
According to this passage, the main duty of the Secretary at War was to stay informed about the military and its supplies.
We are talking about a member of the President's cabinet that does not exist anymore. It used to be the chief of the Department of War. He was responsible for the US military and the Navy, until 1798, in which the Secretary of Nay was created. The last man that occupied this secretary was Kenneth C. Royal, until September 18, 1947.