Explanation: A shunt motor's speed is inversely proportional to the strength of its magnetic field flux. The shunt field winding is responsible for generating this primary magnetic flux. If the shunt field circuit is opened, current can no longer flow through the field winding. This causes the magnetic flux produced by the field to drop drastically (though a small amount of residual magnetism may remain). With a significantly weakened magnetic field, the motor must increase its speed dramatically to generate the necessary back-EMF to oppose the applied voltage. This can lead to a dangerous "runaway" condition where the motor's speed exceeds its safe operating limits. Therefore, opening the shunt field causes the motor to speed up. Options A, B, and D are incorrect because a change in field flux directly impacts motor speed, and a decrease in flux causes an increase in speed, not a slowdown or stop.