What does it mean that federal law is superior to state law?
federal laws makes more sense than any state laws.
federal laws were passed first, before state laws.
states cannot make laws that conflict with federal laws.
states cannot make laws that conflict with local laws.

I think its c States cannot make laws that conflict with federal laws

What it means is that when the federal government makes a law, the states can not make another law that goes against that law. If the states make a law that is unconstitutional, then the federal government has full jurisdiction to deem the law unconstitutional and veto the law in a sense. It is just like the Georgia law that governor Eugene talmage made that Georgia would not integrate. This went against the law that separate but equal was illegal, so the federal government went against it and removed Talmage from office.