High Gain Comparator Circuit
A high gain comparator circuit is designed to compare two input voltages and produce a digital output based on their relative magnitudes. It generally consists of an operational amplifier (op-amp) with positive and negative feedback.

Comparator Input and Output
In this scenario, one input terminal of the comparator circuit is connected to ground, while a sinusoidal voltage is applied to the other input. The output of the comparator will be a digital signal that switches between two voltage levels, typically high and low.

Working Principle
The op-amp in a comparator circuit is designed to have a very high gain, which means even a small difference in input voltages can cause a significant change in the output. When the sinusoidal voltage crosses the ground reference, the output of the comparator will change state.

Explanation of Option D: Square Wave
The output of the comparator will be a square wave because of the following reasons:

1. When the sinusoidal voltage is positive and exceeds the ground reference, the output of the comparator will switch to a high voltage level.

2. As long as the sinusoidal voltage remains above the ground reference, the output will stay at a high voltage level.

3. When the sinusoidal voltage crosses the ground reference and becomes negative, the output of the comparator will switch to a low voltage level.

4. As long as the sinusoidal voltage remains below the ground reference, the output will stay at a low voltage level.

5. This process repeats for each cycle of the sinusoidal waveform, resulting in a square wave output.

Characteristics of a Square Wave
A square wave has the following characteristics:

1. It has two voltage levels, a high voltage level (typically Vcc) and a low voltage level (typically ground or 0V).

2. The transition between the high and low voltage levels is instantaneous.

3. It has equal durations of high and low voltage levels.

4. The frequency of the square wave is the same as the frequency of the input sinusoidal waveform.

5. The duty cycle of the square wave is 50%, meaning the high and low voltage levels are equal in duration.

Therefore, in this scenario, the output of the high gain comparator circuit connected to a sinusoidal input will be a square wave (option D).