Compared to a carnivore, an herbivore needs relatively few pounds of

(A) Herbivores produce protein faster than carnivores. NOT MENTIONED

(B) A pound of carnivore protein has more food value than a pound of herbivore protein. NOT MENTIONED

(C) The impact of people on plant resources would be much less if they substituted chicken for tuna in their diets. CORRECT
Chickens feed on plants (terrestrial), insects
Tunas feed on smaller fish, seaweed (non-terrestrial)
Therefore, the replacement of chicken alleviates human's impact on plant resources.

(D) Carnivores’ diets consist of more plants than meat. NO MEAT INFORMATION MENTIONED

(E) Chickens are a cheaper source of food than tuna. IRRELEVANT

This one is a clear bouncer...someone please help!!

Posted from my mobile device

This question doesn't give the GMAT vibe. It must be from the part of the LSAT universe which is not relevant for GMATers.

Between B, C, and D

B. There is no mention of the food value of carnivores or herbivores in the stimulus - This is clearly out of scope

C. This is utterly nonsense from the GMAT exam prep perspective - because external knowledge about chickens and Tuna is required - Chickens are omnivores, but Tunas are carnivores, so if humans shift their diet from C (Omnivores) to T (Carnivores) they will save plant resources (Which the chickens consume) - Meh, the logic is correct but at what cost.

D. There is no mention of the ratio of meat to the plants in a herbivore - even if there is a supposed correlation between the two, it need not be the same magnitude for every animal on the planet. Ex. Elephants vs goats - both are herbivores with great differences in their diets. Also, newborn herbivore babies will have high meat to plant ratio because they would mostly be feeding on their mother's milk - This is ALSO out of scope

The winner is C, but with presupposed external knowledge