Final answer:
In a UML class diagram, the elements included are the class name, fields, and methods. The correct element not contained in a UML class diagram is the object names.
Step-by-step explanation:
An UML (Unified Modeling Language) diagram is a standardized way to visualize the design of a system. UML diagrams are used primarily in software engineering to depict the structure and behavior of software systems. When constructing a UML class diagram, which is one of the many types of UML diagrams, it is common practice to include certain elements such as the class name, along with its fields (attributes) and methods. However, UML class diagrams do not typically represent the actual instances of a class, known as objects. Therefore, the correct answer is d. the object names.