The correct answer is A. Field controlled D.C. motor.
An open-loop control system is a system in which the control action is not dependent on the output of the system. This means that the control action is determined solely by the input signal, without any feedback from the output.
A field controlled D.C. motor is an example of an open-loop control system. The field current of the motor is controlled by a potentiometer, which is set by the operator. The output of the motor is the torque, which is not measured by the system. Therefore, the control action (the field current) is not dependent on the output (the torque).
B. Ward leonard control is a type of closed-loop control system. In a closed-loop control system, the control action is dependent on the output of the system. This means that the control action is adjusted in order to maintain a desired output.
C. Metadyne is a type of generator that is used in power systems. It is a synchronous generator, which means that the output frequency is synchronized with the input frequency.
D. Stroboscope is an instrument that is used to view moving objects in slow motion. It does this by flashing a light at a frequency that is synchronized with the object’s motion.