Your question seems incomplete or incorrect because the functional dependency you have given here also gives AB as a Candidate key. Anyways, now if we talk about Prime, we will always talk about a particular attribute. In your question, C, D, and E are prime attributes. {C, D, E} will be a set of prime attributes. There should not be much confusion I think. Jointly Prime attributes will also be prime because you can separately define these by Armstrong rules.