From Top to bottom: C, B, D, A. That's the correct matchups (:
The Rocky Mountains are described as younger because the Rocky's are really two mountain chains. The older mountain chain started billions of years ago, and the younger mountain chain pushed the older mountain chain up more, only 60 millions years ago.
Speaking from experience, the answer is not D.
I believe the answer is E. d41 because it goes a11, A12, b21, B22, c31, C32, then it goes lowercase d41 and then the next one is going to upper case D42.
Hope this helps.