Which statement presents a sound argument? The new video game system is so popular that stores may have trouble keeping them in stock. If you won't buy me that game, then don't buy me anything. Everyone we know has the new video game system; we should get one too. Playing video games will rot your brain and make you stupid.

Probably the first sentence: "The new video game system is so popular that stores may have trouble keeping them in stock."

Best answer would be:

" The new video game system is so popular that stores may have trouble keeping them in stock"

This is the only argument that relies on a solid fact. It is hard to disprove popularity; if it's clearly being sold out (because a lot of people are lining up outside of GameStop/BestBuy/etc. or it's being sold out online), then that's solid evidence.

" If you won’t buy me that game, then don’t buy me anything" is not a strong argument because it relies on feelings. There's no incentive to buy the speaker anything. It also seems immature.

" Everyone we know has the new video game system; we should get one too" relies on peer pressure. There is no reason why one should follow the crowd; you will live if you do not buy that new video game system.

" Playing video games will rot your brain and make you stupid" is also not really factual. Video games have been around for a while; if they really were dangerous, they would have been canned a long time ago.