Happy to help. We're trying to weaken the argument.
Pratt believes that because bats are shy, rarely bite, and the majority of bats do not have rabies, the fact that some bats do have rabies doesn't justify getting rid of bats in buildings where people are. We need to select an answer choice that weakens his argument and helps us to make the case that we should still get rid of bats in buildings where people are.
(B) says, "Rabid bats are less mobile than other bats but are much more aggressive."
(B) weakens Pratt's argument because it tells us that rabid bats are much more aggressive than other bats. From this, we know that even if bats rarely bite and most bats do not have rabies, this suggests that the bats that do have rabies bite more frequently, weakening Pratt's position.
Does this make sense? Let us know if you have any other questions!