The correct answer is: The speed of motor will be reduced.
When the field of a D.C. shunt motor gets opened while motor is running, the speed of the motor will be reduced. This is because the field current is reduced, which causes the magnetic field to weaken. The weakened magnetic field allows the armature to rotate faster, which reduces the speed of the motor.
Option A is incorrect because the speed of the motor will be reduced, not increased.
Option B is incorrect because the armature current will increase, not decrease.
Option C is incorrect because the motor will not attain dangerously high speed. The motor will only attain dangerously high speed if the field current is increased, not decreased.
Option D is incorrect because the motor will not continue to run at constant speed. The speed of the motor will be reduced when the field of the motor is opened.