Answer: D. Each ray is perpendicular to its wavefronts, so the angles to the normal are $\alpha$ and $\beta$ Frequency is unchanged at the boundary, so $\lambda_2/\lambda_1=v_2/v_1$ From Snell’s law, $n_1\sin\alpha=n_2\sin\beta$ and $n=c/v$ giving $v_2/v_1=n_1/n_2=\sin\beta/\sin\alpha$
A: reverses the incident-to-transmitted ratio.
B: uses the angle ratio instead of the sine ratio.
C: treats the marked angles as angles between rays and the boundary, rather than between wavefronts and the boundary.
Remember: the angle between a wavefront and the boundary equals the angle between its ray and the normal.