Two waves of the same frequency have opposite phase when the phase angle between them is

The correct answer is B. 180°.

Two waves of the same frequency have opposite phase when the crest of one wave coincides with the trough of the other wave. This happens when the phase angle between them is 180°.

Option A is incorrect because 360° is a full cycle, so the waves would be in the same phase.

Option C is incorrect because 90° is a quarter cycle, so the waves would be in quadrature phase.

Option D is incorrect because 0° is the same phase, so the waves would be in phase.