When the EU Has Exclusive Competence in a Particular Area

When the EU has exclusive competence in a particular area this means that:

A) Member States are prevented from acting unilaterally or collectively in that area, irrespective of whether or not the EU has already acted, unless the EU has explicitly authorised them to implement certain measures.
B) Member States are allowed to act within the limitations imposed by the Treaties as long as the EU is inactive or ceases to be active
C) Only the EU is allowed to act but this is subject to the principles of subsidiarity and proportionality.