Angular frequency is related to time period by the formula W = 2*pi/ T, where W is the angular frequency and T is the time period. Angular frequency is expressed in radians per second.
The time period of oscillation is the time taken for a body to go through one oscillation. It is expressed in seconds.
Here, the tip of the tuning fork goes through 540 complete vibrations in 0.8 seconds. The time period for one oscillation is T = 0.8/540 = 1.48*10^-3 seconds.
The angular frequency is 2*pi/T = 2*pi*540/0.8 = 4241.15 radians per second.