This question is based on an actual GMAT questions, which goes something like "the resulting flow pattern, with crests and troughs that remain stationary, although the air that forms them is moving rapidly, is known as standing waves".

The subject of the 2nd clause is "this group of naturalistic landscapes" which is singular. Hence the verb should be "reflects" and not "reflect". Thus A and B are eliminated.

Next, we want to show the contrast between how simplistic the result looks while the effor that went into the production of the work was complex. The construction "X seems simplistic although the strokes that form them are quite complex" is parallel. C is the right answer.

D is wrong because "brushstrokes that forms" contains a subject-verb agreement error.
E is wrong because it needlessly adds "that" before "reflects", spoiling the connection between the subject of the sentence "this group of naturalistic landscapes" and the verb "reflects".

OA is C

Explanation below:

The sentence after the semicolon has the subject of "this group of naturalistic landscapes"

group - singular
landscapes - plural

RULE: subject should not come inside a prepositional phrase (so it is not "landscapes", as "of naturalistic landscapes" is the prep. phrase)

now see the explanation below with a 2-3 split

arora007 wrote:Among the wall paintings being analyzed by the art critic are
landscapes that characterize art from the Pure Land tradition;
this group of naturalistic landscapes, with mountains and trees
that seem simplistic although the brushstrokes that form them
are actually quite complex, reflect a deep appreciation for
nature that was inspired by Taoist ideals.

mountains and trees that seem simplistic although the
brushstrokes that form them are actually quite complex, reflect - Subject and Verb doesn't agree. Verb should be "reflects" as due to singular subject group

mountains and trees that seem simplistic although they are
formed by brushstrokes that are actually quite complex, reflect - Subject and Verb doesn't agree. Verb should be "reflects" as due to singular subject group

mountains and trees that seem simplistic although the
brushstrokes that form them are actually quite complex, reflects - Perfect sentence

seemingly simplistic mountains and trees although the
brushstrokes that forms them are actually quite complex, reflects - Subject and Verb doesn't agree. Verb should be "form" as due to plural subject "brushstrokes"

seemingly simplistic mountains and trees although they are
formed by brushstrokes that are quite complex, that reflects - "reflects is not the main verb as it comes after the "that", hence it is incorrect

IMO C