A Kangaroo's forelimbs are well developed -> help to climb into mother's pouch
Ancient marsupial lions had only their forelimbs developed -> must have needed to climb to their mother's pouch
Some connection between forelimbs development and climbing mother's pouch, saying former should lead to the laterAnswer evaluation:
(A) All animals that are born after a short gestation period are born with some parts of their bodies underdeveloped.
-- 'All' looks extreme, we are dealing with only 2 animals here and the argument is not primarily concerned about short gestation period
(B) Well developed forelimbs would have been more advantageous to ancient marsupial lions than well developed hind limbs would have been.
-- The argument does not talk about the advantages of forelimbs and hind limbs. Out of scope
(C) If the newborn marsupial lion did not climb into its mother’s pouch, then paleontologists would be able to find evidence of this fact.
-- Whether the marsupial lion actually climbed into the pouch is not discussed in the argument and hence is out of scope. The conclusion just talks about the need to do so
(D) Newborn marsupial lions that crawled into their mothers’ pouches could not have done so had they not had only their forelimbs developed at birth.
-- This could have been a possible assumption, had it not had 'only' in it. On negating, this becomes not only forelimbs developed but also other parts maybe. This answer choice doesn't yield the required result on negation, hence incorrect
(E) Newborn marsupial lions would not have had only their forelimbs developed if this development were of no use to the marsupial lions.
-- CORRECT ANSWER
. This explains that the forelimbs were developed for some purpose. [From Argument climbing into mother's pouch is this purpose].
If said that the lions would have had their forelimbs developed even if they were of no use, we cannot say that marsupial lions needed to climb to mother's pouch [conclusion of the argument]
Conclusion is attached on negating the sentence, hence this is the Best Answer