The correct option is (b) inversely proportional to their diameters

To explain I would say: It is the ratio between the velocities of the driver and the follower or driven.

Let d1 = Diameter of the driver,

d2 = Diameter of the follower,

N1 = Speed of the driver in r.p.m., and

N2 = Speed of the follower in r.p.m.

∴ Length of the belt that passes over the driver, in one minute

= π d1.N1

Similarly, length of the belt that passes over the follower, in one minute

= π d2 . N2

Since the length of belt that passes over the driver in one minute is equal to the length of belt that passes over the follower in one minute, therefore

π d1.N1 = π d2 . N2

∴ Velocity ratio, N2/N1 = d1/d2.