The correct answer is C) 125 psi. According to the regulations, the relief valve on a fire pump must be set at 25 psi above the pressure necessary to maintain the required fire hose streams. The required pressure for fire hose streams is typically around 100 psi. Therefore, the relief valve must be set at 25 psi above 100 psi, which is 125 psi. The other options are incorrect because A) 50 psi is too low, B) 75 psi is too low, and D) 150 psi is too high for the relief valve setting.