A toroidal solenoid has 3000 turns and a mean radius of 10 cm. It has a soft iron core of relative permeability 2000. What is the magnitude of the magnetic field in the core when a current of 1 A is passed through the solenoid?

The magnetic field in the core is given by $B=\mu nI$

When $\mu$ is the permeability of soft iron and n is the number of turns per unit length of the solenoid. 

Now ${\mu }_r=\frac{\mu }{{\mu }_0}$ and $n=\frac{3000}{2\pi r}=\frac{3000}{2\pi \times 0.1}$

$\therefore B={\mu }_r{\mu }_{0}nI=2000\times 4\pi \times {10}^{-7}\times \frac{3000}{2\pi \times 0.1}\times 1=12T$

Hence the correct choice is (d)