Answer:
B (read below, D is probable.)
Step-by-step explanation:
The Supreme Court can "check" a law created by Congress to ensure the branch doesn't "gain too much power" and the IRS must enforce such laws. When using the definition of separation of powers, enforcement and creation are two separate and distinct powers, thus separation of powers. (D)
Although, the question is somewhat vague hinting that the concept of separation of powers is only referring to the three branches of the federal government. In this case (using the definition of rule of law where each citizen must follow the said tax law, of which the IRS is enforcing) the correct answer would be B. Clarification from your teacher could be useful...