If you read my answer for Option A, I've used the term "In ER model". In RDBMS model, there is No such thing as Weak Entity or Multivalued Attributes... These are present in ER model. So, In ER model, A Weak Entity doesn't have any Key. When we make a Table(Relation) corresponding to this Weak enetity in RDBMS model, then We make Key by merging the partial key and primary key of strong entity.