Normal full-term babies are all born with certain instinctive reflexes

The right answer here should be D. As with any question on the GMAT, the first task is to identify what the question wants us to do. In this case, it is a structure based CR question, in which we are being asked to mimic the given argument.

In order to do this well, it helps if we can classify the argument in terms of x,y,z. This allows us to ignore the specifics of the situation and focus on the argumentative logic alone. For this question, the logic is:

X (normal 3 month babies) have Y (no more reflexes)
This thing doesn't have Y (still reflexes), hence it is not X

A - X causes Y, this is not X, hence it wont cause Y. This is not the same logic. OUT

B - X (Apes) have Y (inability to talk), this thing is X (Suzy is ape), hence it will also have Y (she cant talk). This is not the same logic either. OUT

C - X (humans) have Y (social skills), this thing has Y (social skills) and is therefore.... Z? (normal?). This is vastly different logic. OUT

D - X (opossums) have Y (abdominal pouches), this thing doesn't have Y (this animal has no pouch), hence it is not X (opposum). This logic is an exact match and therefore CORRECT

E - X (some trees) have Y (shedding pattern), this thing doesn't have Y (not shed), therefore it is not... Z? (normal?). Like in C, this logic is far from the logic used. OUT

One final thing to note: Options C and E used the word "normal" since the original stimulus used it as well. This is a trap. When you are asked to understand the structure of the argument, remember that the GMAT can use similar sounding content to lead you down the wrong path. Don't fall for it!

- Matoo