The correct answer is D) A bypass or recirculating line led back to the source of suction supply. When a turbine-driven main feed pump needs to operate at shut-off head or less than 20% of its rated capacity, it is necessary to have a bypass or recirculating line that leads back to the source of suction supply (e.g., the feed water tank or deaerator). This recirculation line prevents the pump from overheating by allowing some of the pumped liquid to be diverted back to the suction side, rather than being discharged against a closed or throttled valve. The other options, such as throttling the steam supply valve (B) or the liquid discharge valve (C), would not be effective in preventing the pump from overheating under these conditions. A bypass or recirculation line (D) is the correct solution to ensure the pump is not operating at a severely reduced flow rate, which could lead to overheating and potential damage.