George: Throughout the 1980s and early 1990s, hardly anyone learned ballroom dancing.
Why is it that a large number of people now take ballroom dancing lessons?
Boris: It’s because, beginning in 1995, many people learned the merengue and several related ballroom dances. Because these dances are so popular, other ballroom dances are now catching on.
Boris’s response to George is most vulnerable to criticism because it fails to
(A) show that the people who learned the merengue are the same people who are now interested in other ballroom dances
-- This doesn't link disinterest in ballroom dancing in early decades to later ones (B) explain why ballroom dancing was so unpopular before 1995
-- George's question is not why ballroom dancing was unpopular, rather how it gained sudden popularity. Huge difference! 
(C) relate the merengue to the forms of dancing that were more prevalent before 1995
-- There is no such discussion in the passage(D) account for the beginning of the revival of interest in ballroom dancing
-- CORRECT, Boris merely states that merengue gained popularity in 1995, but doesn't explain "how and why", a question that George is actually asking(E) demonstrate that all types of ballroom dancing are currently popular
-- We are not concerned about all types of ballroom dancing, rather the absence of any before 1990.The answer is
D