Which of the following is NOT an article?

The correct answer is "D" because "in" is a preposition, not an article. Articles are words used to define a noun as specific or unspecific. In English, there are two types of articles: definite and indefinite. The word "the" is a definite article that refers to a specific noun, while "a" and "an" are indefinite articles that refer to a nonspecific noun. For instance, "a cat" refers to any cat in general, while "an apple" refers to any apple, but neither specifies which apple. On the other hand, "in" does not perform this function; it is used to indicate a relationship between the noun and another word in the sentence, typically showing location or position. Thus, identifying "in" as not being an article is correct.