Step-by-step explanation
The refractive index of a medium (n) is equal to the speed of light (c) divided by the velocity of light through the medium (v)

so
Step 1
a) for Diamons

b) fuzed quarts

Step 2
now,compare:

due to c is constant, the velocity 2 is greater ( c is divide by a smaller number)
hence, the speed of light in diamonds is ..
a.lower than that in quartz.
I hope this helps you