Choose the article that correctly completes the sentence. She is ___ best…

Concept — A superlative adjective (best, tallest, most careful) picks out exactly one member of a stated group, so the noun phrase it heads is uniquely identifiable. English marks a uniquely identifiable noun phrase with the definite article "the". The indefinite articles "a" and "an" do the opposite job — they present one unspecified member of a class — and the zero article belongs with plural or uncountable nouns used in a general sense.

Application

1. The word right after the blank is "best", the superlative form of "good".

2. The phrase "in the class" states the group inside which the comparison is being made.

3. A superlative inside a stated group singles out one person, so the noun phrase is definite and takes "the": She is the best student in the class.

Cross-check

• "a best student" and "an best student" clash with the uniqueness that the superlative has already established; "an" additionally needs a following vowel sound, while "best" begins with the consonant sound /b/.

• Leaving the slot empty does not work either: "best student" is a singular countable noun phrase, and English requires a determiner in front of it.

• The same pattern shows up in "the fastest runner", "the most useful tool" and "the tallest building" — superlative preceded by "the".

Answer — the. Complete sentence: She is the best student in the class.