Step 1: Observe the structure of the sentence and check for parallelism. The phrase “pair of red trousers” is followed by a similar construction, so it should maintain the same grammatical pattern. To avoid repetition while keeping the structure parallel, the correct substitute is “pair of green ones.”
Step 2: The word one is singular, whereas it refers back to a plural noun (trousers). Using the singular form breaks number agreement and disrupts the parallel structure of the sentence.
Step 3: Therefore, the grammatical error occurs in part (C), which uses one instead of the required plural form ones.