The correct answer is C) a rudder order is given by the control system. Under normal operating conditions, the rudder is hydraulically locked unless a rudder order is given by the control system. This is because the rudder is typically controlled by a hydraulic system, and the hydraulic lock keeps the rudder in a fixed position until a command is received from the control system to move the rudder. The other options are incorrect because: A) the variable stroke pump being off-stroke would not unlock the rudder, B) the manual trick wheel is used for emergency steering, not normal operation, and D) an electric power system failure would not automatically unlock the rudder, as the hydraulic system should still be operational.