My answer is
(A). It took me 01:14.
This is probably one of few questions that could have been answered by looking at options only. I usually spend 3-5 seconds previewing options before reading the sentence.
(A) Very decent answer. Keep for now.
(B) "socially" is an adverb and cannot directly modify a noun ("flaws"). Eliminated.
(C) "whom" can only modify a person not a thing. Eliminated. Note that "whose" can modify a thing as well as a person.
(D) "socializing", a present participle, can be used to modify a noun. This sentence is grammatically sound. The only difference between (A) and (D) is the choice of the modifier for flaws. Keep for now.
(E) See (C). Eliminated.
Now, I had to choose between (A) and (D), at which point I read the complete sentence for the first time.
Yet it did
not help.
Both are decent sentences. No test takers should be expected to have read "American Nerd". Nerds actually have the reputation of being socially awkward in dealing / socializing with others, an outside knowledge that might argue for (D). But "socializing flaws" are "social flaws".
Finally, I picked (A) only because it is more common to read "social flaws".