In retrospect, the answer C looks like an inference. It does not look like an assumption(unstated premise/evidence).
The argument clearly states that the seed-eating bird spends more time eating than does the nectar-eating bird.
Tha answer C says that The time it takes for the nectar-eating bird to eat a given amount of nectar is not longer than the time it takes the seed-eating bird to eat the same amount of seeds
Thus, C is just a paraphrase of the above sentence in the argument. A paraphrase can be an inference but not an assumption.
Do let me know your views. Probably, I might be 'out of scope'.
Birds need so much food energy to maintain their body temperatures that some of them spend most of their time eating. But a comparison of a bird of a seed-eating species to a bird of a nectar-eating species that has the same overall energy requirement would surely show that the seed-eating bird spends more time eating than does the nectar-eating bird, since a given amount of nectar provides more energy than does the same amount of seeds
The argument relies on which one of the following questionable assumptions
(A) Birds of different species do not generally have the same overall energy requirements as each other
(B) The nectar-eating bird does not sometimes also eat seeds
(C) The time it takes for the nectar-eating bird to eat a given amount of nectar is not longer than the time it takes the seed-eating bird to eat the same amount of seeds
(D) The seed-eating bird does not have a lower body temperature than that of the nectar-eating bird
(E) The overall energy requirements of a given bird do not depend on factors such as the size of the bird, its nest-building habits; and the climate of the region in which it lives
Thnx & Rgds,