Lyme disease is caused by a bacterium transmitted to humans by deer ticks. Generally, deer ticks pick up the bacterium while in the larval stage by feeding on infected white-footed mice. However, certain other species on which the larvae feed do not harbor the bacterium. If the population of these species increased, more of the larvae would be feeding on uninfected hosts, so the number of ticks acquiring the bacterium would likely decline.

Which of the following would it be most important to ascertain in evaluating the argument?

Conclusion: If the population of the other species increased, the number of ticks acquiring the bacterium would decline.

Premises: The other species do not harbor the bacterium. The ticks generally acquire the bacterium while feeding in the larval stages.

Assumption: That the ticks will feed on the the other species if given the opportunity to do so.

Prediction: The correct answer will address the assumption and strengthen or weaken the conclusion. It will show whether the ticks actually will feed on the other species.

To understand the answer choices more easily, rephrase them without the word whether.

(A) The populations of the other species on which deer tick larvae feed are found only in the areas also inhabited by white-footed mice. Doesn't help us determine whether the ticks will feed on the other species.
(B) The size of the deer tick population is not currently limited by the availability of animals for the tick's larval stage to feed on. In other words, the ticks have plenty of food; the number of ticks is not being held back by the amount of food available. Thus, more food will NOT lead to more ticks. Implication: Any larvae that eat the uninfected food will not increase the total population of ticks but will instead DECREASE the number of ticks getting infected, STRENGTHENING the conclusion that the number of infected ticks will decrease. CORRECT.
(C) The infected deer tick population could be controlled by increasing the number of animals that prey on white-footed mice. Outside the scope. The argument is not about how the population can be controlled but about whether the population would increase.
(D) The deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed. Tempting, but incorrect. The argument states that the way the ticks generally get infected is by feeding in the larval stages. This is a premise of the argument and cannot be disputed. Any answer choice that discusses other ways the ticks can get infected is irrelevant. Even if ticks can get infected as adults, this is not the way ticks generally get infected, so who cares? A word of advice: an answer choice that attacks a premise will not be correct. The correct answer will address the assumption, which in this case is that the ticks will feed on the other species.
(E) Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmit to humans. Out of scope. The argument isn't about other bacteria.