The correct answer is B) facilitate water removal. The main reason for having a low suction line on the fuel oil service or settling tanks is to allow any water that has settled at the bottom of the tank to be easily removed. This helps maintain the quality of the fuel by preventing the water from being drawn into the fuel system. Having the suction line positioned low in the tank ensures that the water, which is denser than the fuel, can be drained off without losing a significant amount of usable fuel. The other options are incorrect because: A) preventing loss of suction during rough weather is not the primary purpose of the low suction line; C) decreasing suction head on the pump is not the main reason; and D) increasing the amount of fuel available for use is not directly related to the purpose of the low suction line.