He is famous . . . . . . . . his learning.

Prepositions are used to show the relationship between words in a sentence. In this case, the preposition "for" is the correct choice to show the reason or cause behind someone's fame. When we say "He is famous for his learning," it means his fame is attributed to his learning ability. The preposition "for" is commonly used to indicate the reason or cause behind something. Therefore, Option C: for is the correct choice in this context.