Can someone explain the logical structure of the premise/conclusion in the question and how that coincides with the correct answer?

^I have the same question as above. Got the question correct due to ABC diagramming, but wanted to know if that is the best method.

The passage sets up a scenario where we have a general rule and then a specific application of that rule. We are told that all members of a class (normal full term babies) possess a certain trait ( losing reflexes by month 2). We are then told that since this individual lacks that trait (still has reflexes ), this individual isn't a member of the aforementioned class. We are looking for the same structure: all members have a trait, X doesn't have this trait, so X isn't a member. This is a match form D. D tells us that all members of a class have a pouch, and that since X doesn't have a pouch, it's not a member of the class opossum.