Final answer:
A mental representation for a set of things in the world treated equivalently for some purpose is called a concept. The best example within a concept is known as a prototype. Therefore, the correct answer to the question is D. Both A and C.
Step-by-step explanation:
A mental representation for a set of things in the world that are treated equivalently for some purpose is known as a concept. This means that, in cognitive psychology terms, the answer is A. Concept. A concept involves categorizing and combining various pieces of information into a cognitive structure, which helps us see relationships among elements and keeps information organized in our minds. A prototype is closely related to the idea of a concept as it is considered the best example within that concept, such as a Golden Retriever being the prototype for the category of dogs when that is a person's most familiar or early experience with dogs.
However, when considering the completeness of the answer, we should acknowledge the role of prototypes in forming and understanding concepts. Thus, the answer would be D. Both A and C, encompassing both the abstract concept and its best representative prototype.