a b c d

b

$\text{From Ampere's Circuital law inside the wire}$ $\int B.dl = \mu_0 I_{enclosed} = \mu_0 \frac{ I\times \pi r^2}{\pi R^2} $ $\Rightarrow B.2\pi r = \mu_0 \frac{ I\times \pi r^2}{\pi R^2} $ $\Rightarrow B = \frac{\mu_0 I r}{2\pi R^2}$ $\Rightarrow B \propto r $ $\text{Value of B at 1mm is }\frac{B}{2}$