The correct answer is: D. no article
The word “intelligent” is an adjective that describes the boy. Adjectives do not usually take articles.
The article “a” is used before singular nouns that refer to a general or unspecified person or thing. For example, “I saw a boy walking down the street.”
The article “an” is used before singular nouns that begin with a vowel sound. For example, “I saw an apple on the table.”
The article “the” is used before singular nouns that refer to a specific person or thing that has already been mentioned or is known to the listener. For example, “I saw the boy who stole my bike.”
In the sentence “He is intelligent boy,” the word “intelligent” is an adjective that describes the boy. Adjectives do not usually take articles, so the correct answer is “D. no article.”