Final answer:
Among the choices given, 'User interface screen layouts' is not considered a design model, while package diagrams, sequence diagrams, and activity diagrams are all UML design models used in software engineering.
Step-by-step explanation:
The item among the choices provided that is NOT considered a design model is c. User interface screen layouts. Design models are typically concerned with the structure and behavior of the system. Options a, b, and d, namely Package diagrams, Sequence diagrams, and Activity diagrams, are all types of Unified Modeling Language (UML) diagrams which are commonly used in software engineering to illustrate different aspects of software design. In contrast, user interface screen layouts are concerned more with the visual design and user experience aspects, not the underlying software architecture or process flows.