Answer:
c. states from passing laws that contradict federal law.
Explanation:
The supremacy clause in the constitution gives the federal law power over state laws.
The clause traces its origin to the 1819 McCulloch v. Maryland case of the supreme court, which gave a principle for an outline of the later would be; Article four, Section two of the U.S constitution.
By a state passing laws that contradict the federal laws, then, the state shall be going against the supremacy clause.
In case of a conflict between the federal laws and the state laws, then the federal laws take the day.
This clause has got the importance of guaranteeing national union out of the address of a possible reason for difference between the federal and state governments.
It is the responsibility of the president of the US to uphold the supremacy clause since he is the one to ensure that the power of the states does not tower that of the federal government.