I'm guessing the "correct" "answer" is F, because it's asymmetrical; my "tricky-dar" tells me that's the answer they expect. But, like you said, who the hell really knows?!

The point of MENSA, I think, is to make a few people feel better about themselves. And they can whip out their allegiance in internet arguments, in an attempt to impress people -- "I'm a member of MENSA, so I know what I'm talking about." I've seen it happen. The guy was patently fucking wrong.