Final answer:
The element not associated with Use Case Diagrams is b. Aggregate relationship, which is actually a part of class diagrams within UML and represents a 'whole-part' relationship.
Step-by-step explanation:
The modeling element among the options given that is not associated with Use case Diagrams is b. Aggregate relationship. In Use Case diagrams, which are a part of UML (Unified Modeling Language), the elements you'll typically find are the actor, use case, include relationship, and extend relationship.
An actor represents a user or another system that interacts with the system being modeled. The use case is a description of a set of actions that the system should perform in collaboration with one or more actors. The include relationship is a directive that allows one use case to include the functionality of another. The extend relationship is similar but represents an optional functionality that extends the behavior of a use case.
On the other hand, an aggregate relationship is a term that is more commonly associated with class diagrams within UML and not use case diagrams. In class diagrams, aggregation is a specific type of association representing a 'whole-part' relationship, such as a classroom containing desks.