Answer is D. Light travels more slowly in Medium A than in Medium B because Medium A is dense.
When light travels from a medium of low refractive index to a medium of high refractive index, it slows down in the latter and bends towards the vertical. The extent to which the light ray bends is directly related to the medium's refractive index (or the optical density).
Lets understand the given passage.
A rectangle labeled Medium A is drawn on the left. A ray of light travels through air and strikes the surface of medium A making an angle of 50 degrees with the vertical. The light ray bends towards the vertical as it travels through Medium A and makes an angle of about 15 degrees with the vertical. A rectangle labeled Medium B is drawn on the right. A ray of light travels through air and strikes the surface of medium B making an angle of 50 degrees with the vertical. The light ray bends towards the vertical as it travels through Medium B and makes an angle of about 25 degrees with the vertical.
The light bends more in medium A than medium B. Hence medium A is optical denser than medium B.
