c
Mean radius of a Rowland ring, \(r=15\, cm =0.15\, m\)

Number of turns on a ferromagnetic core, \(N=3500\)

Relative permeability of the core material, \(\mu_{r}=800\)

Magnetizing current, \(I=1.2\, A\)

The magnetic field is given by the relation:

\(B=\frac{\mu_{r} \mu_{0} I N}{2 \pi r}\)

Where, \(\mu_{0}=\) Permeability of free space \(=4 \pi \times 10^{-7} \operatorname{Tm}\, A ^{-1}\)

\(B=\frac{800 \times 4 \pi \times 10^{-7} \times 1.2 \times 3500}{2 \pi \times 0.15}=4.48 \,T\)

Therefore, the magnetic field in the core is \(4.48 \,T\)