The most commonly used amplifier in sample & hold circuits is

Option A: A unity gain non-inverting amplifier is correct because the most commonly used amplifier in sample & hold circuits is a unity gain non-inverting amplifier. This configuration provides a stable output with no amplification (gain = 1), which is ideal for sample & hold applications where the input signal is maintained without distortion.

Option B: A unity gain inverting amplifier is incorrect because inverting amplifiers flip the signal, meaning the output is inverted with respect to the input. This is not typically desired in sample & hold circuits, where the signal should remain in phase with the input.

Option C: An inverting amplifier with a gain of 10 is incorrect because amplifying the signal in this way would distort the original input, which is undesirable in sample & hold circuits where the purpose is to maintain the exact value of the sampled signal.

Option D: An inverting amplifier with a gain of 100 is incorrect because, like Option C, amplifying the signal with a high gain (such as 100) would lead to a distorted output, which is not suitable for sample & hold circuits.

Conclusion: The correct answer is Option A: A unity gain non-inverting amplifier because this configuration is commonly used in sample & hold circuits due to its ability to maintain the input signal without inversion or amplification.