Bunuel
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
When the future is indicated from the point of view of the past. Simple future is not used. Instead conditional is used.
(A) would gradually undermine it
Instead of the pronoun "it", we need the noun. The use of "it" leads to confusion. Eliminate this option.
(B) to be a gradual undermining of it
We need conditional "would" here. Eliminate this option.
(C) would be a gradual undermining of slavery
It correctly uses conditional "would" for a future event indicated from the point of view of the past. The use of noun "slavery" is correct. Keep this option.
(D) to gradually undermine slavery
We need conditional "would" here. Eliminate this option.
(E) gradually undermining it
We need conditional "would" here. Eliminate this option.
Option C is the correct answer.