The correct answer is (B).
(B) Principle (Identify/Assumption)
Step 1: Identify the Question Type
This question asks for a principle. Since the principle "underlies" the argument, that means you need to identify the principle, rather than apply it. Break the argument down into conclusion and evidence, and pinpoint the assumption. Then rephrase the argument in broader, more general terms.
Step 2: Untangle the Stimulus
The phrase "this shows that" in the last sentence indicates the conclusion. Everything mentioned before shows how the conclusion is supported, and thus is evidence. The conclusion says that some dogs descended from more recently domesticated wolves than other dogs did. Before reaching that conclusion, the biologist presents the general belief that dogs descended from domesticated wolves. Then comes the primary piece of evidence: Some dog breeds are genetically closer to wolves than they are to most other dog breeds.
Step 3: Make a Prediction
The biologist presents quite a scope shift. The evidence offers a variance in how closely certain dogs are related to wolves. The conclusion then discusses when certain wolves were domesticated. The biologist assumes that a dog that is more closely related to wolves than to other dog breeds must have descended from a more recently domesticated wolf. Stated as a rule, this general assumption serves as the principle underlying the argument.
Step 4: Evaluate the Answer Choices
(B) expresses the principle just right. Since certain dog breeds are genetically closer to wolves than to most other dog breeds, this principle allows for the conclusion that those breeds came from wolves that were more recently domesticated.
(A) distorts the author's assumption in two ways. First, the author is concerned with dog breeds more closely related to wolves than to other dog breeds, not those more closely related to wolves than other breeds are. Second, the author takes a close relation to wolves as evidence that the wolves from which a breed originated were more recently domesticated, and not vice versa.
(C) provides more detail about the comparison in the author's evidence, but it doesn't represent the principle on which the author builds the argument. Indeed, denying the statement here would make the author's argument even stronger. This choice says that there are no dog breeds genetically closer to wolves than they are to all other dog breed. The biologist's evidence is simply that some breeds are closer to wolves than to most other dog breeds; he'd be happy to find a breed closer to wolves than it is to any other breed.
(D) is a meaningless tautology. Saying that a dog breed more closely related to wolves than another dog breed is therefore more closely related to wolves than the other dog breed is adds nothing to the argument. This choice doesn't address any aspect of when domestication appeared or what it indicates.
(E) is Extreme. It's unclear if dog breeds that are more closely related to each other than they are to wolves came from wolves domesticated long ago. The author only asserts that some dogs are more related to wolves than to most dogs. The author doesn't present any evidence about dogs that are more closely related to most other dogs than to wolves.
