d
Frequency of fork \(1=200 \mathrm{Hz}=\mathrm{n}_{0}\)

No. of beats heard when fork \(2\) is sounded

with fork \(1=\Delta n=4\)

Now we know that if on loading (attaching tape) an unknown fork, the beat frequency increases (from \(4\) to \(6\) in this case ) then the frequency of the unknown fork \(2\) is given by, \(\mathrm{n}=\mathrm{n}_{0}-\Delta \mathrm{n}=200-4=196 \mathrm{Hz}\)