Answer:
B. Separation of Powers.
Step-by-step explanation:
It is very important that the powers of the executive branch and of the President of the USA are defined. If it is not defined, we would have to answer the question "Who should be the commander-in-chief?" Congress? Chief Justice? The Joint Chiefs of Staff? The mayor of Washington D.C.? By clearly establishing the responsibility of the president as commander-in-chief , the Constitution´s fathers followed the principle of the separation of powers.