I didn't start studying for the DAT yet but I have the kaplan book and I looked at the problem. My guess is..
It's obiously not A, B, or C. If you look at choice E, the rounded parts on the bottom are uneven on either side. The shape in the problem shows that both sides are even so that it why choice E is not the answer. Therefore, you are left with the answer D.
I hope this helps. As i said I haven't started studying for the Dat so this is only my guess. I could be wrong.