Answer: D. Separation of powers
Explanation: The principle of Separation of powers refers to the division of government functions and powers into distinct branches, in order to prevent the concentration of power in one branch and to provide for checks and balances (Where all branches oversee, limit and check each other).
The U.S. Constitution is based on this principle, as it divides the government into three branches of government: The judicial, the Executive and the Legislative. So, the fact that Congress is the only branch with the duty of introducing and pass laws, while the President is the only one with the task of enforcing federal laws, illustrates this principle of separation of powers.