He drives at a speed of 90 miles . . . . . . . . hour. A. a B. an C. the D. no article

The correct answer is D. No article.

The word “hour” is a noun that refers to a period of time equal to 60 minutes. It is not a countable noun, so it does not take an article.

The other options are incorrect because they are all articles. An article is a word that is used to introduce a noun. The articles “a” and “an” are used before singular countable nouns, and the article “the” is used before singular and plural countable nouns that are specific or unique. In this case, the noun “hour” is not a countable noun, so it does not take an article.