GMAT Sentence Correction (SC) | Re: Ryunosuke Akutagawa s knowledge of the literatures of

Here we have three items: A, B, and C. In this case, Europe, China, and Japan. There is no need for the 'that' before Japan. That eliminates (A) and (B).

Next, we have the subject 'knowledge', which is singular. Therefore the verb is 'was.' And just like that, we are at answer (D) .