The correct answer is A. Ward Leonard control.
Ward Leonard control is a method of controlling the speed of a DC motor by varying the voltage applied to the motor. This is done by using a variable resistor, called a rheostat, to control the amount of current that flows through the motor. By varying the resistance of the rheostat, the voltage applied to the motor can be varied, which in turn controls the speed of the motor.
Rheostat control is a method of controlling the speed of a DC motor by using a variable resistor, called a rheostat, to control the amount of current that flows through the motor. This method is simple and inexpensive, but it is not very efficient. When the rheostat is used to control the speed of a motor, there is a significant amount of electrical energy wasted as heat.
Neither of the above methods is the correct answer.