Final answer:
The concept of a red apple being always considered a fruit represents a graded structure, making it the best example according to prototype theory.
Step-by-step explanation:
According to prototype theory, a graded structure occurs when some members of a category are seen as more central or “prototypical” than others. In this context, the best example of a graded structure would be “a fruit is always a red apple”. The concept of fruit varies widely, but red apples are often considered a central or prototype member of this category because people commonly think of apples when they think of fruit. This is a prime example of a graded structure, where apples serve as a more typical member of the fruit category as opposed to fruits like kiwis or avocados, which may be seen as less typical. In prototype theory, these typical examples are believed to be processed more easily and quickly in the human brain.
Responses such as “an athlete is only a professional Olympian” or “a bird is only a sparrow or a pigeon” do not represent graded structures as they are too specific and not universally accepted as central examples of their categories.