Which of the following is an example of an antonym?

An antonym is a word that has the opposite meaning of another word. In this case, "cooperative" refers to someone willing to work with others or assist. On the other hand, "stubborn" describes someone who refuses to change their mind or cooperate. These two words represent opposing traits, making them a correct example of antonyms.

The other choices, while they may represent shades of meaning or be similar, do not convey direct opposites. For example, "beautiful" and "pretty" are synonyms; they mean nearly the same thing. Similarly, "difficult" and "hard" are also synonymous terms. Finally, "old" and "ancient" describe things that are not new but do not represent a clear opposition in meaning as "cooperative" and "stubborn" do. Thus, "cooperative/stubborn" stands out as the example of antonyms.