What Important Principle of EU Law Was Established in the Case

What important principle of EU law was established in the case of 6/64 Costa v ENEL?

A) Direct effect of EU directives.
B) The supremacy of EU law in a situation where there is a conflict between EU law and national law of a Member State.
C) The principle of State liability under which a Member State can be liable for any loss suffered by an individual as a result of that State