The correct answer is D) fully opened. Spur gear pumps should be operated with the discharge valves fully opened. This is to ensure the pump operates at its maximum efficiency and prevents excessive back pressure from building up in the system. With the discharge valves fully opened, the pump can maintain its rated flow and pressure, which helps to avoid damage to the pump components. The other options are incorrect because: A) slightly opened would create too much back pressure, B) throttled would restrict flow and reduce efficiency, and C) halfway opened would also create unnecessary back pressure.