Which statement about word classes is correct?

• A. Word classes are categories of words defined by what they do
• B. Word classes are all open to new words
• C. Open word classes include prepositions and determiners
• D. Lexical field defines a set of words that share a topic

Answer: (A)

Word classes are defined by what they do in a sentence—their function or syntactic behavior. This means each class is a category based on how its members operate grammatically: nouns name things, verbs express actions or states, adjectives describe nouns, adverbs modify verbs/adjectives, and so on. That functional basis is what makes the statement correct: word classes are categories defined by what they do.

It's helpful to add that some classes are closed and don’t easily take new members (like prepositions and determiners), while others are open and readily admit new words (like nouns, verbs, adjectives, and adverbs). A lexical field, meanwhile, groups words by topic or meaning, which is a semantic concept rather than a grammatical class issue.