It would be the job of the "c. the president of the United States" to uphold the supremacy clause of the United States Constitution, since this ensures the power of the federal government over the states.
The judiciary has in its highest court, the Supreme Court, the role of interpreting the American Constitution and ensuring that it is fulfilled, this includes the clauses of supremacy. That is, we can say that the Supreme Court is the main guardian of the Constitution and this is one of the tasks of the judiciary power.
