Conclusion: Thus, the current average weight of Galapagos Tortoises is undoubtedly at least partially attributable to changes associated with the Galapagos' environmental conditions
Which of the following is an assumption on which the argument depends?
(A) There are no types of vegetation on the Galapagos Islands that are known to benefit from dry conditions. -> vegetation was not discussed. Irrelevant.
(B) There were about as many tortoises on the mainland five thousand years ago as there are on the Galapagos Islands today. -> Irrelevant. We need to talk about weight of tortoises.
(C) The mainland's environment has not changed in ways that have resulted in tortoises on the mainland growing to be 25 kilograms lighter than they were five thousand ago. -> So, it is eliminating any other possibilities of weight reduction at mainland's tortoises. So, we left with G-Island environment to be contributor of weight increase of tortoises. Let's keep it.
(D) The soil on the South American mainland, unlike that on the islands, lacks important nutrients that help tortoises survive and grow heavy in a dry environment. -> It may lead to open other possibilities of weight reduction at South American mainland. And, we don't want to do it. Incorrect.
(E) The 25-kilogram weight difference between the tortoises on the Galapagos Islands and those on the mainland is the only difference between the two populations. -> Irrelevant.
So, I think C.