atleast one Multivalued attribute implies there will be atleast two tables in relational model...
Now candidate key for tables for multi valued attribute will be (CK_of_E + multi_valued_attribute_itself)
Assuming E had only single attribute as one of the CKs then relational model will have only one simle CK.
By simple CK they meant CK should be composed of only 1 atrribute not more than 1. CKs of Multivalued attribute tables will have two attributes.