What does a NOT gate do to its input?

A NOT gate, also known as an inverter, performs a fundamental operation in digital logic. Its primary function is to invert the input signal. When the input is 0, the NOT gate outputs 1, and when the input is 1, the output is 0. This means that the output of the NOT gate is always the opposite of the input value.

In the context of the provided choices, the correct response accurately describes this behavior: it outputs 1 when the input is 0. This inversion ability is a crucial aspect of digital circuits, allowing them to perform logical operations and control signals in systems such as computers and other electronic devices.