The correct sentence is D. Joaquin is the quietest person in my class.
The above sentence is correct because it uses an appropriate superlative degree of an adjective as compared to other sentences.
Adjectives refer to the words which modify or qualifies the noun or pronoun.
Whereas, superlative adjectives involve comparison of two or more nouns in the highest degree. They are formed by adding -est to the word. "most" and "least" are used with two or three syllable words.
Hence, the superlative form of word quiet is quietest. Therefore, it is in accord with the last option.
