Here is my thought on this.
C

Option A here has the word conception which I believe is a wrong usage.
Options B & E looks like Irish politics is a critique of colonial economics. Additionally Option E does not give a contrasting opinion with the usage of "also it was".
Option D also seems to describe a colonial economics as a satirical commentary on Irish politics. Similar to option E, "which was" does not contrast what Swift intended.

Here is the official answer from Veritas:
Correct answer: (C).

The semi-colon in the original sentence should alert you that Sentence Construction is being tested here. The original uses the semi-colon correctly (with independent clauses on either side), but there are errors within those independent sentences. "Swift's conception of X was as Y" is wordy and unidiomatic. It is more clear and direct to say that "Swift conceived of X as Y." Eliminate (A), (D), and (E) because they use other constructions. (B) eliminates the semi-colon and creates a Sentence Construction error. We should find an independent clause after ", but". Answer (C) is correct because it takes two phrases that are in opposition to each other ("a satirical commentary on Irish politics" and "a critique of colonial economics") and subordinates the second to the first, tightening the structure. What follows the semi-colon corrects the Verb Form error in the original, using a verb in the simple past tense instead of the past perfect. Choice (D) is also guilty of a Modifier error; as written "which was" modified "politics" incorrectly. Choice (E) commits another, more egregious Sentence Construction error, introducing "...it was" as the subject of a clause without creating an independent clause after "colonial economics...".