Correct option is C
In a negative feedback amplifier, the input impedance is increased when the signal sampled is a voltage (option a), which is also referred to as "voltage shunt feedback" where the feedback signal is taken in parallel with the input voltage.
Explanation:
Voltage sampling (shunt feedback): When the feedback signal is a voltage sampled in parallel with the input, it effectively increases the input impedance because the feedback network appears as a high impedance to the input signal.
Current sampling (series feedback): Conversely, if the feedback signal is a current sampled in series with the input, it decreases the input impedance.
