Hi @JayDee8732. This is a Weaken question ("most seriously undermines the researcher's reasoning"). First, let's examine the stimulus.
The Researcher presents an argument, which proceeds as follows: P: It's commonly believed that species belonging to the same biological order descended from a single ancestor. P: The Researcher compared the genetic pattern found in 3 rodent species and also in 13 nonrodent mammals. P: He found that while rats and mice are genetically quite similar, guinea pigs are as different from mice as mice are from some other, nonrodent species. C: Thus, guinea pigs stem from a different ancestor [than do rats and mice].
Let's look carefully at how the Researcher's reasoning proceeds. He really reaches his conclusion based on one fact: the fact that mice and guinea pigs are as different from each other (genetically) as mice are from some other nonrodent species.
Fine. Let's weaken that fact! This is what answer choice (B) does. It says: "Some pairs of species not having a common ancestor [e.g., mice and, let's say, a nonrodent like a giraffe] are genetically more similar to each other than are some pairs [of species] that DO have a common ancestor [let's say, mice and guinea pigs]."
If (B) is true, then just because the mice are genetically quite different from the guinea pigs does not, alone, establish the conclusion that mice and guinea pigs stem from a separate genetic ancestor.
