This is an apple. . . . . . . . . apple is red. A. a B. an C. the D. no article

The correct answer is: D. no article

The indefinite article “a” or “an” is used before singular nouns that refer to a general or unspecified thing. In this case, the speaker is referring to a specific apple, so the indefinite article is not necessary.

The definite article “the” is used before singular nouns that refer to a specific or unique thing. In this case, the speaker is not referring to a specific apple, so the definite article is not necessary.

No article is used before singular nouns that refer to a general class of things. In this case, the speaker is referring to a specific apple, so no article is necessary.