Hi Mundasingh,

You're right - the conclusion states that since marsupial lions' forelimbs were well developed, those lions must also have needed to climb into their mothers' pouches.

One assumption here, as you point out, is that the forelimbs would not have been developed for some other purpose (like catching prey). Another assumption -- stated in E -- is that the forelimbs wouldn't have been developed for no reason at all.

The negation test is actually helpful here. Negate the assumption in E and you get a statement that says: "Newborn marsupial lions WOULD have had only their forelimbs developed if this development were of no use to them." If this statement were true, then the conclusion would not hold: The author's argument relies on the idea that forelimbs are developed for the particular purpose of climbing into a mother's pouch.

One reason to be careful when using the negation test with this specific question, though, is that choice E is already structured as a double negative! When negating a negative, you just have to take extra care to preserve the meaning of the answer choice

Hope that helps!