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 for the removal of any water that may accumulate at the bottom of the tank. This water can be drained off through the low suction line, preventing it from being drawn into the fuel system and potentially causing issues with the engine or other fuel-related equipment. The other answer choices are incorrect because: A) is not the primary reason, as the low suction line is more about water removal than maintaining suction during rough weather; C) decreasing suction head is not the main objective, as the low suction line is designed to access the lowest part of the tank; and D) the low suction line is not intended to increase the amount of fuel available for use, but rather to remove unwanted water.