The correct answer to this question is:
(d) Averaging amplifier
An averaging amplifier is a type of op-amp circuit where the output voltage is equal to the average of the input voltages. It takes multiple input voltages and produces an output voltage that represents the average of those inputs. This is often used in applications where you need to average multiple signals, such as in signal processing or data analysis.
In summary, the averaging amplifier is the one where the output is the average of the input voltages.