Correct Answer: C
Explanation:
Electromagnetic wave is a transverse mode. It has the electric field E is perpendicular to the direction of propagation. This electric field can accelerate the electron on the surface and impart momentum to the point of reflection. This should not happen as it does not move. Therefore, the reflected wave should have an electric field in the direction opposite to the direction of the original E field. Therefore, there is a change of 180 degrees.