Concept:Words like "trousers" are always plural because they are made of two matching parts, so we count them using "a pair of".
Explanation:The noun "trousers" has no singular form in normal usage.
It refers to one garment that covers both legs separately.
Therefore, "a trousers" and "one trousers" are incorrect because they treat a plural noun as singular.
"Plenty trousers" is also wrong because "plenty" must be followed by "of" (i.e., "plenty of trousers").
To describe exactly one such item, English uses the fixed expression "a pair of trousers".
This is similar to "a pair of scissors" or "a pair of glasses".
Answer:C. a pair of trousers.