The core concept here is understanding the primary purpose of a dam in a river system. The correct answer is 'C. To prevent flooding' because dams are commonly used to control the flow of water and prevent excessive flooding. Option A is incorrect because a dam typically slows down the water flow, not increases it. Option B, while true for some dams, is not the main function for all dams and thus is not the best answer. Option D is also a function of some dams but not the primary purpose for all dams.