Dear student, first of all we will find the time period and 1/T is frequency.

So, we know that the time period of any oscillating thing is given by

T = 2 × pi × sqrt( I/M*B)

Now, frequency, f =1/T

=>(1/2*pi)×sqrt( M*B/I)

Now in the above equation 2*pi, I amd M are constant. Therefore,

f is directly proportional to B.

Now applying the formula -

((f2/f1) - 1)×100 =((B2/B1) - 1)×100

In the question its given that the increase is by 21% so putting this value on the right hand side

((f2/f1) - 1)×100 =10%

So the frequency of magnetic needle will be 10%.

All the best 

Hope this helps