IMO B. Though I'm not sure

For the solution to work,we need to know whether there is a direct link between the DT population and the white foot mice availability. Only if there is a direct linkt between DT and the WFM, the suggested solution will work, but if there is no direct link between these two (that is, DT and WFM), the solution will not work. So we need to know whether there is a link between these two things (i.e the DT population and WFM availability) to complete the argument.

Choice B does this and hence is the correct option.

Let me know if this does not help!

I would go for D. Whats the OA ?

plus 1 for D

I was thinking answer would be C. But checking the argument again I find I agree with others
correct answer could be B
Conclusion: If we increase the population of other species, then the number of deer ticks getting infected with disease may decline.

Assumption would be : Do other species exists enough in number to feed deer ticks

(A) Whether populations of the other species on which deer tick larvae feed are found only in the areas also inhabited by white-footed mice.
--If there are little far, won't affect the argument

(B) Whether the size of the deer tick population is currently limited by the availability of animals for the tick's larval stage to feed on.
--This close to the assumption we can make from argument, so i feel its correct

(C) Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white-footed mice.

--The conclusion is whether we can decrease the deer ticks getting affected by L disease by making them feed on other uninfected species. We are not bothered about the deer population , we want to decline the number of dicks getting affected. We are not told in argument that deer ticks die when they gets disease

(D) Whether deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.
-- this is little attrative, But the argument gives the condition 'when the deer ticks feed on uninfected host'. If infected dicks have fed on deer then deer might have got affected so is no more uninfected

The argument was tough to understand as I didnt know deer ticks are different from deer

(E) Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmit to humans.
--other bacteria is not part of conclusion.

This argument is based on the assumption that deer's population won't increase. The right answer B has actually hit on the assumption and questions the gap between conclusion and assumption.The conclusion doesn't say anything about population of deer.

D doesn't contribute to the discussion. Lets say even if the situation is right, even then no one can guarantee that deer population will be huge to affect humans.