c
Along the equatorial line, magnetic field strength

\(B = \frac{{{\mu _0}}}{{4\pi }}\frac{M}{{{{({r^2} + {l^2})}^{3/2}}}}\)

Given: \(M=4\, \mathrm{J} T^{-1}\)

\(r=200 \,\mathrm{cm}=2 \,\mathrm{m}\)

\(\ell=\frac{6 \,\mathrm{cm}}{2}=3\, \mathrm{cm}=3 \times 10^{-2}\, \mathrm{m}\)

\(\therefore B=\frac{4 \pi \times 10^{-7}}{4 \pi} \times \frac{4}{\left[2^{2}+\left(3 \times 10^{-2}\right)^{2}\right]^{3 / 2}}\)

Solving we get, \(B=5 \times 10^{-8}\) \(tesla\)