The correct answer is A. Supreme court.
Supreme court has a power to determine whether a law is constitutional.
The supreme court of United States, is given the power to check the congress and president their actions.
If the actions of a president are not in the constitution then it can tell the congress the U.S constitution is being violated.
All powers are not entitled to supreme court but they are limited by the two branches of the government.
For example, if a president nominates a justice to the court, then his/her approval for nomination must be voted in the senate.
