One boy is wearing . . . . . . . . black trouser. A. a B. an C. the D. no article

The correct answer is D. no article.

The word “trouser” is a plural noun, so it does not take an article. In this case, we are not referring to a specific pair of trousers, but rather to any pair of trousers. Therefore, we do not need to use an article.

The other options are incorrect because they are all articles. The article “a” is used before singular nouns that refer to a general thing or person. The article “an” is used before singular nouns that begin with a vowel sound. The article “the” is used before specific nouns that have already been mentioned or are known to the listener.

In this case, we are not referring to a specific pair of trousers, so we do not need to use an article.