a and d. A is quite obvious. For d, consider a analogy-light travelling along x direction with its plane perpendicular to x axis. Consider 2 rectangular glass blocks in its path so that light falls on them normally. The ref. Indices of the blocks vary linearly in the y and z direction respectively, so that when light emerges, the wave front is no longer perpendicular. B and c are not possible as a continuous set of points in the direction of wave can never have the same phase.
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Moderation Team
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