The correct answer is: B. This will make armature to take heavy current, possibly burning it.
When the shunt field circuit of a DC shunt motor suddenly opens, the field current decreases to zero. This causes the magnetic field to collapse, which in turn causes the armature current to increase. The armature current increases because the back emf (electromotive force) of the motor decreases. The back emf is proportional to the speed of the motor, so when the field current decreases, the speed of the motor increases. The increase in speed causes the armature current to increase even more, and this can cause the armature to overheat and burn.
Option A is incorrect because something will happen to the motor. The field current will decrease, which will cause the armature current to increase. This can cause the armature to overheat and burn.
Option C is incorrect because the motor will not necessarily run at excessive speed. The speed of the motor will increase, but it may not reach an excessive speed.
Option D is incorrect because the motor will not necessarily run at very slow speed. The speed of the motor will decrease, but it may not reach a very slow speed.