Explanation:

A comparator is an electronic device that compares two voltage input signals and determines which one is larger. It has two input terminals, one for the input voltage and the other for a reference voltage. The output of the comparator is a digital signal indicating whether the input voltage is higher or lower than the reference voltage.

Comparator Types:
There are several types of comparators, each with its own characteristics and applications. Some of the common types include:

1. Voltage Follower:
A voltage follower, also known as a unity gain amplifier, is a comparator circuit where the output voltage follows the input voltage. It has a very high input impedance and a very low output impedance. The output voltage is equal to the input voltage and there is no voltage gain.

2. Digital to Analog Converter (DAC):
A digital to analog converter is a device that converts digital signals into analog signals. It is not a type of comparator but rather a separate device used to convert digital information into analog voltage levels.

3. Schmitt Trigger:
A Schmitt trigger is a comparator circuit that incorporates positive feedback to provide hysteresis. It is commonly used for noise filtering and signal conditioning. The output of a Schmitt trigger changes state only when the input voltage crosses certain threshold levels, resulting in a more stable and less noisy output.

4. Voltage Level Detector:
A voltage level detector is a comparator circuit that detects whether an input voltage is above or below a certain threshold level. It generates a digital output signal indicating the voltage level of the input signal.

Conclusion:
Based on the given options, the correct answer is option 'D' - Voltage Level Detector. This is because a voltage level detector is a type of comparator that compares the input voltage with a reference voltage to determine if it is above or below a certain threshold level.