Explanation: A sine wave is the fundamental waveform of alternating current (AC) and is crucial in radio theory. Option B accurately describes its geometric generation: if you imagine a point rotating uniformly around a circle, its vertical (or horizontal) projection onto an axis over time will trace out a perfect sine wave. This illustrates its smooth, periodic variation in amplitude. Option A is incorrect because a sine wave's voltage is constantly varying, not constant. Option C is wrong because a sine wave follows the trigonometric sine function, not the tangent function, which has infinite discontinuities. Option D is incorrect; a sine wave's polarity changes in a perfectly predictable and regular manner, not randomly.