Speed of light in each block, fastest to slowest: B, C, A.
Work Step by Step
According to Snell's Law (page 526), the amount of bending is greatest when the speed change is the greatest. Light slows down when it enters each of the blocks from air. The change in angle is smallest in picture B, meaning light travels the fastest there. The angle changes the most in picture A, meaning light slows down the most when it enters block A.