Here is the conclusion of the argument.

If the population of these species(species which even after eating infected white-footed mice do NOT pickup bacterium) increased, more of the larvae(deer ticks species) would be feeding on uninfected hosts, so(Conclusion indicator) the number of ticks acquiring the bacterium would likely decline.

In other words, we can say that the argument is concluding by stating that if the population of the species which do not pickup bacterium even after eating infected white-footed mice is increased, then most of the infected white-footed mice will be eaten by these species. And the rest of the food is somewhat better for the deer ticks species and this can reduce to some extent the number of deer ticks acquiring bacterium which inturn reduces the number of humans getting Lyme disease.

Now look at the answer choices – which support this conclusion - reason for deer ticks acquiring bacterium reduction / humans getting the Lyme disease reduction

1. Consequences from carrying bacterium have no impact on the number of humans getting Lyme disease. Rule out this option.
2. Cases of Human’s contracting Lyme disease thru contract is white-footed mice have no impact on reducing the number humans getting Lyme disease. Rule out this option.
3. This looks supporting the conclusion because – deer ticks(a species) feeds only once while at larval stage and at this stage if the feed available is NOT contaminated, then they do not pick up the bacterium and there by reduce the number of humans getting Lyme disease. Possible ans.
4. One host animal being the source for many deer tick larvae to pick up the bacterium --> indicates even with one infected animal many deer ticks can pick up bacterium --> leading to more humans getting Lyme disease. Weakening the conclusion. Rule out this option.
5. Deer ticks getting other bacterium is irrelevant because the argument is taking abt the bacterium leading to Lyme disease. Rule out this option.

My pick is C.

here is a link detailing the explanation on how to solve this question type. (Note that there is slight change in the wordings of the conclusion.)

http://www.manhattangmat.com/forums/post2001.html

Very similar, but different post at tough-cr-lyme-disease-81209.html

+1 C, reasons as stated above.

Please post the OA after sufficient discussion, this question is 2 years old and no OA/OE.
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cheers
Mayank
My GMAT Journey: 720 + dare = 760

i guess it is C

Yes C it is. Thanks for the explanations guys.
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Consider KUDOS if you feel the effort's worth it

As bacteria in larvae stage feeds only once chances are it would feed on the non infected animals as they are more in number hence C.