An article is a word used to modify a noun. We use articles to point out or refer to nouns in conversations and writing. There are two articles, which are “a,” and “the.” If you are referring to a specific noun, use the definite article “The.”
Articles specify grammatical definiteness of the noun and in some languages extending the volume or numerical scope. Examples would be, The fox devours the apples we leave in the yard would refer to a specific fox and A fox devours the apples we leave in the yard would refer to any fox. These two small words are used frequently, and even though they are small, they are significant.
An article indicates that a noun will follow. A is correct. B is not correct because adjectives modify nouns. C is not correct because conjunctions connect words and phrases in a sentence. D is not correct because: 1) a subject usually (but not always) comes before a predicate; and 2) the question asked for the purpose, not the usage, of articles.