Charge q is uniformly spread on a thin ring of radius R. The ring rotates about its axis with a uniform frequency f Hz. The magnitude of magnetic induction at the centre of the ring is

B = μ₀I/(2R)
where I is the current. The charge q rotates with frequency f Hz. Therefore, the time period is T = 1/f.
The current I is given by the charge flowing per unit time, I = q/T = qf.
Substituting this into the equation for B, we get
B = μ₀qf/(2R)