We have a strange argument here. The author is trying to prove that teaching short kids to reach high shelves will prevent them from becoming short adults. The author thinks this is the case because short kids are bad at reaching tall shelves, but tall kids are good at it, and short kids are more likely to become short adults.
This is one of the most flawed arguments I have seen on the LSAT- and I had trouble parsing what the author's argument. The author seems to think that the ability to reach shelves is what determines one's height as an adult. What is far more likely (and less insane) is that reaching shelves is a function of height as a kid, and that short kids tend to be short their entire life, and tall kids tend to be tall their entire life. That is, the author assumes that shelf-ability causes height, while in reality, it is more likely that height causes shelf-ability.
This is a good match for D. The author falsely assumes that one thing causes another, when in reality they are only related to each other, but the author is incorrect about which thing causes the other.