He is the . . . . . . . . boy in class.

The boy mentioned in the sentence is a good boy. However, there is mention of only one boy in the sentence. Hence, option A 'best' is the most appropriate option which means there is no other boy who is better than him in the class. There is a superlative degree of comparison and a definite article is needed to complete the sentence.
Option B- better would be used if there is a comparison of the boy with someone.
Option C- good cannot be used as there is comparison with other boys.