Choice C is the best answer. The previous question asks what potential criticism might be made of Volkov's testing of Hodick and Sievers's model. The answer, that a central element of that model wasn't corroborated by Volkov's measurements, is best supported in the last paragraph: "This made the trap close without any direct touch to its trigger hairs (while they didn't measure calcium levels, the current likely led to increases)." Because the physical touch to the hairs figured in Hodick and Sievers's model, it can be said that Volkov's decision to apply an electrical current directly to the plant means that he failed to corroborate a central element of their model.
Choices A, B, and D are incorrect because the cited lines don't support the answer to the previous question. Instead, they summarize the basic agreement of Volkov's work with Hodick and Sievers's model (choice A) and describe steps in Volkov's experimental design that are related to the application of an electrical current but don't directly address the omission of the central element of the physical touch to the hairs (choices B and D).