Correct option is (b) 268 mm
Explanation: Given : l = 2.5 m ; r = 250 mm = 0.25 m ;
Since the wheel makes 50 torsional oscillations in 170 seconds, therefore frequency of oscillation,
n = 50/170 = 5/17 Hz
Let kG = Radius of gyration of the wheel
We know that frequency of oscillation (n),
5/17 = r/2πkG √g/l = 0.25/2πkG√9.81/2.5 = 0.079/kG
kG = 0.079 x 17/5 = 0.268 m = 268 mm.