This is the best answer. The last sentence of P2 presents an interesting behavior observed in cannibal tadpoles. Sometimes they do eat siblings, even though other times they don’t. This is a fact that an explanation of kin recognition will have to take into account. Whether we propose inclusive fitness theory or something else to explain the cannibal tadpole’s behavior, our explanation will need to address why the tadpole sometimes eats its siblings.

The author doesn’t provide any evidence concerning the behavior of cannibals in “most” species. So (B) wouldn’t make sense as the author’s purpose.

Although the rest of the discussion of tadpoles supports the author’s account of kin recognition (inclusive fitness theory), the sentence we’re asked about does not. It contains an example that shows sometimes a tadpole will still eat its siblings, which isn’t something inclusive fitness theory would predict.

This can’t be the purpose, because the author does try to offer an explanation of the cannibal tadpole behavior. The rest of the discussion in P2 shows that the author believes inclusive fitness theory can explain the behavior. Although there is a certain kind of behavior that inclusive fitness theory doesn’t necessarily explain, this doesn’t imply that it’s impossible to explain the cannibal tadpole behavior.

The author doesn’t suggest that the last sentence of P2 is more relevant than the previous discussion in P2. In fact, the previous discussion in P2 shows how cannibal tadpole behavior in general can be explained by inclusive fitness theory. That discussion is equally or more relevant than the behavior described in the last sentence, because the author’s overall point in the passage is to show how inclusive fitness theory can explain at least some instances of kin recognition.