Hi Jonathan,
First, in case you missed it, here's a link to the beginning of the discussion of this question, including a breakdown of the answers.
viewtopic.php?f=775&t=1662
Parallel reasoning questions are designed to be time consuming by their very nature/length. Fortunately, there usually aren't many of them in any given LR or RC section, and there often are quicker ways of approaching these questions.
In this case, the key is to first be very clear and precise on the differences between the ways that U.S./Canadian common law and classical Roman law treat blackmail.
U.S./Canadian common law explicitly prohibits what otherwise would be two legal actions separately, that become illegal (blackmail) when combined. It's legal to reveal private information that harms an individual if it's true, and it's legal to ask people for money, but it's not legal to threaten to reveal private harmful information if not given money (i.e. blackmail). The first part of Answer D, the prohibition of felons owning guns matches this perfectly because it's not illegal to own a gun (by itself) and it's not illegal to be a felon (by itself), but the two combined become illegal.
You mentioned the triangular nature of the blackmail relationship mentioned in Passage A, but that is the author's explanation for a coherent theory of blackmail rather than an actual feature of how blackmail is treated in the U.S./Canadian common law, and therefore doesn't really come into play in this question.
For Classical Roman law, blackmail is prohibited because the underlying conduct itself of revealing harmful private information is generally prohibited with very narrow exceptions made for "a legitimate purpose" and when it "dealt with a matter that the public authorities had an interest in having revealed" (lines 54-56). The second part of Answer D, making gun ownership illegal for everyone but police and military matches this perfectly because the underlying conduct itself (gun ownership) is generally prohibited with very narrow exceptions made (police and military).