There's a lot going on in this sentence. First, recognize that the pronoun "their" in the beginning of the underlined passage is correct, as it refers to the plural noun "the Whigs." That eliminates (B) and (D).

Next, zero in on the comma. In the underlined passage, you should be able to read around the phrase "hovering nervously," meaning there is no comma separating the clauses "the Whigs had seized upon..." and "Defoe was between upholding...." Since the latter is a complete sentence, a comma should be present. Not only does this eliminate (A), it also knocks out (C) and (D). While both (C) contains commas around the word "nervously," they serve the same purpose as the commas around "hovering nervously" in (A). Choice (E) is the only one remaining, and it is correct.