(b) 50 Hz The magnetic field energy in an inductor is given by,E=21​Li2 The magnetic energy will be maximum when the current will reach its peak value, i0​ , and it will be minimum when the current will become zero.From the above graph of alternating current, we can see that current reduces from maximum to zero in T/4 time, where T is the time period.So, in this case, T/4=5 ms ⇒T=20 ms ∴ Frequency,ν=T1​=20×10−31​=50 Hz