The correct answer is A) secure the valves in the supply and return lines. To prevent the rudder from moving while making a repair on the steering system, the appropriate action is to secure the valves in the supply and return lines. This effectively isolates the actuator from the hydraulic system, preventing any inadvertent movement of the rudder. The other options, while potentially relevant in other situations, do not directly address the need to isolate the actuator and prevent rudder movement in this specific scenario.