This is a very tough question- its pretty uncommon to have to think about ratios and mass/surface area, which I personally find a bit off putting and dangerously close to math.
Since this is a strengthen with necessary premise question, we need to find an answer choice that, if not true, would completely destroy the argument.
We are told that energy needed is proportional to weight, and available energy is proportional to surface area. This means that an animal with a greater surface to weight ratio will be able to move uphill faster than an animal with a small surface area to weight ratio. The argument also tells us that small animals are able to run uphill faster than large ones. This means that, if the author's conclusion is true, these small animals must have a greater ratio than large animals.
C tells us exactly that... mostly. It tells us that the ratio of surface area to weight is smaller for large than for small animals- which is the same as saying the ratio is larger for small animals. IF this were not true, the entire argument would make no sense, so this must be an assumption the argument depends on.
