To solve this problem, we need to understand the functionality of a NOT gate and how it affects an input signal.

1. Understanding the NOT Gate:
   • A NOT gate, also known as an inverter, is a digital logic gate that outputs the opposite (inverted) state of its input. If the input is HIGH (1), the output will be LOW (0), and vice versa.
2. Analyzing the Input Signal:
   • We are given a 1 kHz square wave as the input. A square wave alternates between HIGH and LOW states at regular intervals. Hence, a 1 kHz square wave changes from HIGH to LOW and back to HIGH every 1 millisecond, completing one full cycle in 1 ms.
3. Effect of the NOT Gate on the Input Signal:
   • When the 1 kHz square wave is applied to the input of a NOT gate, the output will be its inversion. Thus, whenever the input is HIGH for 0.5 ms, the output will be LOW for the same period, and when the input is LOW for 0.5 ms, the output will be HIGH.
   • The resulting output will still be a 1 kHz square wave because the frequency does not change; only the phase is inverted.
4. Conclusion:
   • The output of the NOT gate is a 1 kHz square wave signal that is inverted (complemented) concerning the input.

Based on the analysis, the correct answer is: A 1 kHz square wave signal inverted (complemented) with respect to the input.