Classicist: In the ancient Athenian democracy, unlike in any other political system in world history, the legislature consisted of all eligible voters.
Conclusion (Thus)
If the world has ever known true democracy, it was only in ancient Athens.
Evidence
It was only in ancient Athens that the legislature consisted of all eligible voters. Athenian voters made all political decisions by direct vote after public debate in the Assembly.
Evaluate
The conclusion we're trying to prove is a weird type of claim. Suppose I said, . I'm not actually trying to prove she has daisies. I'm saying daisies are the only possible type of flower in her kitchen, but it wouldn't make me wrong if there weren't daisies. I would only be wrong if there were some other type of flower in her kitchen.
So, similarly, we don't need to prove that ancient Athens was a true democracy. We just need to know that every other political system has not been a true democracy.
Since the argument didn't give us a proving mechanism for "If xyz, then not true democracy", the correct answer will have to give us such a rule.
What do we know about all other political systems that would allow us to prove they aren't true democracies? We know that only ancient Athens had a legislature consisting of all eligible voters. So in every other political system, we know that wasn't the case.
Goal
So, we're probably going to see an answer like, .