Answer:
C. Commander in Chief.
Step-by-step explanation:
The President of the United States is a position that gives numerous 'official positions' in the administration. Some of these posts include "Head of State, Chief Executive, Chief Diplomat, and "Commander in Chief" among others.
The role of "Commander in Chief" allows the president to be the supreme military commander in whose hands lies the decision to make any military decision. This includes the protection and defense of the homeland. And by authorizing the use of military force in Vietnam, President Lyndon Johnson was fulfilling his role as the "Commander in Chief" of the Army and Navy. This means that he can command the armies and other military forces to move however he wants, being lawfully given the authorizing power.
Thus, the correct answer is option C.