\(F =\frac{\mu_{0} I ^{2} a }{\pi \sqrt{ b ^{2}+ a ^{2}}}\)
\(\tau= F \cos \theta \times 2 a\)
\(=\frac{\mu_{0} I^{2} a}{\pi \sqrt{b^{2}+a^{2}}} \times \frac{b}{\sqrt{b^{2}+a^{2}}} \times 2 a\)
\(\tau=\frac{2 \mu_{0} I^{2} a^{2} b}{\pi\left(a^{2}+b^{2}\right)}\)
If \(b \gg a\) then \(\tau=\frac{2 \mu_{0} I ^{2} a ^{2}}{\pi b }\)
But among the given options \((1)\) is most appropriate
[$m _{p}=1.6 \times 10^{-27} kg , e =1.6 \times 10^{-19} C$ નો ઉપયોગ કરવો.]