Explanation: A square wave is a fundamental waveform in electronics and digital communication. Option D accurately describes its characteristic shape: it rapidly switches between two distinct voltage levels and maintains each level for a specific duration before abruptly switching to the other. This rapid transition and holding of levels distinguishes it from sinusoidal or triangular waves. Square waves are crucial in amateur radio for clock signals, digital data transmission, and generating specific audio tones. Option A is incorrect because a complete cycle for any periodic waveform is always 360 degrees. Options B and C are incorrect because a square wave can be entirely positive (e.g., 0V to +5V), entirely negative (e.g., -5V to -1V), or symmetrical (e.g., -5V to +5V). Their descriptions are too restrictive and do not define the shape.