The correct answer is: D. Stabilizes its gain
Feedback is a technique used in amplifiers to control their gain. It works by taking a small amount of the output signal and feeding it back into the input in phase or out of phase. This can be used to increase or decrease the gain of the amplifier, or to stabilize its gain.
Increasing the input impedance of an amplifier is not a function of feedback. The input impedance of an amplifier is determined by the design of the amplifier itself, and feedback does not affect it.
Increasing the gain of an amplifier is also not a function of feedback. The gain of an amplifier is determined by the ratio of the output voltage to the input voltage, and feedback can be used to increase or decrease this ratio.
Controlling the output of an amplifier is a function of feedback. Feedback can be used to make the output of an amplifier more stable, or to make it follow a desired signal more closely.
Therefore, the correct answer is: D. Stabilizes its gain