Thanks for the question, and great username, coolbeans! This is a Justify the Conclusion question, also known in some circles as a Sufficient Assumption question. We are looking for an answer that, when added to the argument as a new premise, makes the conclusion inescapably true.
The premises here tell us that ancient Athens is the only political system to date that involved every eligible voter voting directly on the issues. The conclusion is that ancient Athens is the only possible example of a "true democracy". What new bit of information will inextricably link those two claims, so that the latter must be true?
One way to approach these questions is to mechanistically connect the "rogue" elements, which are the things that are not common to the premise and conclusion. "Ancient Athens" is common to both, so we don't need to talk about it in our answer choice. Here, the rogue elements are "every eligible voter" and "true democracy". Find the answer that connects those, and you have a contender! Only answer D makes that connection, and so it has to be the right answer.
If that mechanistic approach doesn't suit you, try it another way: conditionally. The premise is this claim:
AEV (all eligible voters)
AA (Ancient Athens) (in other words, if you are looking at a system in which all eligible voters are directly involved in the process, then you must be looking at ancient Athens, because that was the only one to ever do that)
The conclusion is this claim:
TD (true democracy)
AA (if it is a true democracy, it's ancient Athens)
What would connect that premise to that conclusion, from a conditional standpoint? This would:
TD
AEV
This works because then you would have this conditional chain:
TD
AEV
AA
That would justify (prove) the claim of TD
AA
Whichever approach you take, mechanistic or conditional, you should end up with answer D as a perfect match and the only contender worth considering.
Give that a look and let us know your thoughts about it. What other answer was attractive to you, and why? Knowing more about your thought process will help us to better analyze your process and make useful suggestions for you. Looking forward to hearing more from you!
Adam M. Tyson
PowerScore LSAT, GRE, ACT and SAT Instructor
Follow me on Twitter at
https://twitter.com/LSATadam