The correct answer is answer A ("The executive branch's power to wage war independently of Congress greatly increased").
According to the passage, Vietnam had violated United Nations' International Laws on several attacks towards the United States. In general terms, Congress has to approve military action before the president can send troops to fight. In that sense, the executive branch is dependant on Congress power to manage the military. Here, however, the president is explicitly given all support from Congress, which greatly increases the executive's power to wage war.
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