Quote:
Although British and French abolitionists of the late 1700s predicted that manumission and the blockading of slave ships
would gradually undermine it, slavery continued to be the dominant cultural and economic force in southern America until the end of the American Civil War in 1865.
(A) would gradually undermine it
(B) to be a gradual undermining of it
(C) would be a gradual undermining of slavery
(D) to gradually undermine slavery
(E) gradually undermining it
predicted (past tense) => verb in that clause will be "would + inf verb without to" => B,D,E out.
(A) would gradually undermine it
(C) would be a gradual undermining of slavery
gmat prefer the verb form to noun form but in this case pronoun "it" is ambiguous since there are some singular nouns in the sentence manumission, blockading, slavery, force, America, the American Civil War. If pronoun "it" is the subject of the first clause (right after although) it is fine to refer to "slavery".
Thus the best here is to use "slavery" rather than "it" in order to deliver clear meaning.
Answer: C