Neither the late 18th century composers, such as Mozart or Haydn, nor

OE

Split #1: This subject is a neither P nor Q structure. Whether the first term, P, is singular or plural does not matter at all. All that matter is the second term, Q, which here is “Beethoven, as well as Schubert in the early 19th century.” Of course, the “as well as” is an additive phrase, so does not count as part of the subject. The only relevant subject is “Beethoven,” and of course Mr. Beethoven is singular. We need the singular verb. Choices (A) & (B) & (C) are all incorrect.

Split #2: to describe a single activity, the correct idioms is “known to compose”: these people did not do this one thing, composing suites, but that is not what made them famous overall. The idiom “known for composing” is a correct idiom, but here it illogically implies that not composing suites is precisely what made these men famous: that’s ridiculous! The construction “known by composing” even more illogically suggests that someone else, unnamed, does the composing, or the “not composing”, and this is how we know these men. The construction “known in composing” is not a correct idiom at all. Only (A) and (D) use the correct idiom for this context.

Choice (D) is the only possible answer.