Answer:
Options: (a) and (d)
Explanation:
Measured variable is required for feedback and feed forward as the corrective measures taken by the controller are based on the measured variable as the without the measured variable there won't be any error.
Since, error is the difference between the measured variable and a set standard.
In the feedback control mechanism, evaluation of error is done and then corrective measures are taken according to the error evaluated no matter known or unknown.
Thus option (a) and (d) are correct.