With CR questions, pay particular attention to any arguments that involve the conflation of NUMBERS and PROPORTIONS.

Premises:
- deer ticks pick up the bacterium while in the larval stage by feeding on infected white-footed mice
- certain other species on which the larvae feed do not harbor the bacterium

Conclusion:
- If the population of these other species were increased, more of the larvae would be feeding on uninfected hosts, so the number of ticks acquiring the bacterium would likely decline.

Logical Gap:
- We're assuming that if the NUMBER of other food-species increased, then the PROPORTION of infested food supply would decrease, and thus the NUMBER of infected ticks would decrease. But what if increasing the food supply merely increased the total population of ticks? We might have the same number of infested ticks, even if the proportion of the tick population was smaller.

We need to evaluate: will the overall size of the tick population change?

A. Whether populations of the other species on which deer tick larvae feed are found only in areas also inhabited by white footed mice.
These populations don't need to be found ONLY in areas inhabited by white-footed mice. They just need to share some area, so the ticks could potentially feed on both.

B. Whether the size of the deer tick population is currently limited by the availability of animals for ticks 's larval stage to feed on
Whether the tick population is currently limited by food supply will directly affect whether the overall population will change. Correct!

C. Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white footed mice.
This is outside of the scope of the argument. The cause-effect relationship we care about is: increasing non-white-footed-mouse food -> smaller number of infected ticks.

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.
The chronology of infection is irrelevant.

E. Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmits to humans.
This argument only deals with Lyme disease bacterium. "Any other bacteria" is outside of the scope of the argument.

The answer is B.

The NUMBER v. PROPORTION flaw is a very common type of logical flaw on CR. For more examples, see:

https://www.beatthegmat.com/critical-re ... tml#725229
https://www.beatthegmat.com/i-m-doubting ... tml#551227
https://www.beatthegmat.com/cr-evaluate- ... tml#558393
https://www.beatthegmat.com/total-books- ... tml#680834
https://www.beatthegmat.com/statistics-c ... tml#564609