This question is an inference question where we can direct answer choice to the fact presented from the passage.
The author believes that spirit of the Constitution is supportive of the Congress’s involvement in decision to deploy troops, so more involvement is advocated, which we can gather from the end of the first paragraph as well as the very last sentence of the passage.
Let's get to the answer choice.
A) We are not given with any instance with what the agreement in decisions was.
B) Line 5-8 states that it’s rather hazy in authority.
C) the author advocates that there should be “more” involvement than now, not more than President explicitly.
D) “final authority” is not covered.
E) Considering that Constitution doesn’t deny president’s involvement while suggesting for more involvement from Congress, this is likely to be the answer. Also the very last sentence supports this as well. (Correct)