What are the 3 ways to increase the speed at which the electric motor changes ?

Explanation:

1.By increasing the applied voltage: Though this method is the easiest and cheapest, it is rarely used because a large change is supply voltage is required for relatively small change in speed. Also, large change is supply voltage disturbs the magnetic conditions of the motor.

2.By increasing the supply frequency: Increasing the supply frequency will increase the speed too. However, if the frequency is increase beyond the rated value, the developed torque falls while the speed rises.

3. Constant V/f control method: In this method, V/f ratio is kept constant when varying frequency for speed control. Keeping V/f ratio constant will also keep the stator flux constant and also keeping the torque approximately constant.

i think the answer would be b