hi - you didn't ask an actual question, so i'll just give a quick general discussion of why the correct answer is correct.

* the prompt question is a bit strange: "which of the following would be most important to ascertain?" this is a bit like asking for an assumption, or for something that would weaken / strengthen the argument if it turns out to be true / false.

in general, when you get this prompt question, you should just EXAMINE THE EFFECT ON THE PASSAGE if the given statement is true, and, alternately, if it is false.

for (b):
if this statement is TRUE (i.e., the larval population is currently limited), then increasing the population of other animals will NOT decrease the # of larvae feeding on infected mice. if its population is currently limited in this way, then the larva will continue feeding on all the infected mice on which it currently feeds; it will merely use the additional animals as an opportunity to increase its population, with less competition.

if this statement is FALSE (i.e., the larval population is not currently limited), then increasing the population of other animals WILL decrease the # of larvae feeding on infected mice (assuming that preferences are equal / indifferent). this will be so because the larval population will not expand, and so will diffuse over the existing animals (meaning more ticks on the animals that aren't infected mice, and fewer ticks on the infected mice).

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(a)
the word "only" makes this one irrelevant.
we are of course interested in whether the white-footed mouse inhabits the same regions as do the other species on which the tick feeds - if it doesn't, then there can't be any interplay between them - but exclusivity (as shown by the "only") isn't important one way or the other.

(c)
nothing here has anything to do with anything in the argument.

(d)
if this can't happen, then it still won't be able to happen, so there will be no noticeable effect.
if this can happen, then it will still be able to happen with the same frequency, so there will still be no noticeable effect.

also, notice that the passage says "GENERALLY, the tick picks up the bacterium in its larval stage".
premises don't lie, so, even if the scenario depicted in (d) can happen, it will do so only to a small extent.

(e)
the passage is concerned only with the lyme disease bacterium, so this is irrelevant.