Which option best describes the extension of a term?

• A. The mental content of the term
• B. The usage of the term in language
• C. The concept's referent in reality
• D. The set of all things to which the term applies

Answer: (D)

The extension of a term is the set of all things the term applies to. In logic, terms have two related ideas: extension and intension. The extension is the actual class of objects in the world that fall under the term—like the entire group of dogs, including every dog that exists or has existed. The intension, by contrast, is the mental concept or sense of the term—the features that define what counts as a dog.

So, for a term like “dog,” its extension is the complete set of dogs. If the term has no real-world instances (as with unicorn), its extension would be empty. The other options point to the mental content (intension), or to how we use the term in language, or to a single referent, rather than the aggregate set that the term covers.