B.

The author is assuming a reason without considering any other cause for the same. Had the author stated that , since it is known that the only way bats can locate each other is by distinguishing each others calls and pups do not have that ability then B would not be correct but as this is the only reason that the author has considered, B has to be the answer.

b is correct.

I selected A as i derived that the "facts known about representative members" that the author is talking about is the pubs not being able to recognize moms

Hmm , actually, reading the question again, i can see that the author in fact did indicate that the bat calls are the only means of finding each other. If that is the case, then it hints that the author did in fact consider other possibilities. in which case B can be eliminated.

It's the equivalent of saying that XYZ car is only made in the color red. Bob just bought XYZ car so it has to be red in color.

Yet the answer would be that the author does not consider that Bob could have bought the car in another color. Doesn't make sense.

Hopefully, an expert will come clarify this if i am right to doubt the answer.

I interpreted answer (B) a little differently.

(B) "Establish the validity of one explanation for a phenomenon by excluding alternative explanations"

Answer (B) states that the author seeks to establish the validity of one explanation by excluding alternative explanations. I take this to mean that the author has considered and rejected alternative explanations. The explanation that the author seeks to establish is that bats find one another because each mother bat can recognize the call of her pup.

The passage states that "(1) the bats' calls are their only means of finding one another, and (2) a bat pup cannot distinguish the call of its mother from that of any other adult bat". This implies that the author has considered two explanations for how bats find one another. The alternative explanations are that (1) bats use something other than bat calls to find one another and that (2) bat pups can distinguish between the calls of their mothers and other adult bats.

The author rules out these explanations in favor of the explanation that bats find one another because the mother bats recognize the call of their pups.