If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will

The correct answer is A. Increase.

The back EMF of a DC shunt motor is directly proportional to the speed of the motor. This is because the back EMF is generated by the magnetic field produced by the current flowing through the motor’s armature. The faster the motor rotates, the stronger the magnetic field, and the higher the back EMF.

The other options are incorrect because:

• Option B: The back EMF does not decrease with increasing speed. In fact, it increases.
• Option C: The back EMF does not remain the same with increasing speed. It increases.
• Option D: The back EMF does not become zero with increasing speed. It increases.