A rod with circular cross-section area \(2\,cm ^2\) and length \(40\,cm\) is wound uniformly with \(400\) turns of an insulated wire. If a current of \(0.4\,A\) flows in the wire windings, the total magnetic flux produced inside windings is \(4\,\pi \times 10^{-6}\,Wb\). The relative permeability of the rod is (Given : Permeability of vacuum \(\left.\mu_0=4 \pi \times 10^{-7} \;NA ^{-2}\right)\)

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(C) \(125\)

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\(\phi=\mu_{ r } \mu_{ o } \frac{ N }{\ell} I \times A\)

\(\mu_{ r }=125\)

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