Karim is _ best student in the class.

Explanation

Option A, "a," woulbe used if Karim was one of many best students in the class. Option B, "an," would be used if "best student" started with a vowel sound, such as "an excellent student." Option C, In this sentence, "the best student" refers to a specific student in the class, so the definite article "the" is the appropriate choice. Option D, "no article," is not appropriate in this sentence because it refers to a specific student who is the best in the class.