The correct answer is D. no article.
The sentence “He has always been a very good boy” is grammatically correct, but it is not the most natural way to say this. In general, we do not use articles with general statements about people or things. For example, we would say “He is a good boy” or “She is a beautiful woman,” but we would not say “He is a very good boy” or “She is a very beautiful woman.”
In this case, the article “a” is unnecessary because it does not add any new information to the sentence. The sentence already tells us that the boy is good, so there is no need to specify that he is a good boy.
The article “an” is also unnecessary because it is used before singular nouns that begin with a vowel sound. The word “boy” begins with a consonant sound, so the article “an” would not be correct.
The article “the” is used to refer to specific people or things. In this case, the boy is not being referred to specifically, so the article “the” would not be correct.
Therefore, the correct answer is D. no article.