The correct answer is C) prevent the pump from overheating when operating at shutoff head. A recirculating or bleed-off line is installed on a centrifugal pump to ensure that the pump does not overheat when operating at shutoff head, which is the maximum pressure the pump can generate with the discharge valve closed. At shutoff head, the flow through the pump is zero, so there is no cooling effect from the fluid moving through the pump. The recirculating line allows a small portion of the pumped fluid to be diverted back to the pump inlet, which creates some flow through the pump and prevents it from overheating. The other answer choices are incorrect because they do not accurately describe the primary purpose of a recirculating line on a centrifugal pump.