A cylindrical conductor of radius $R$ is carrying a constant current. The plot of the magnitude of the magnetic field $B$ with the distance $d$ from the centre of the conductor, is correctly represented by the figure:

Using Ampere's Circuital Law:Inside the conductor ($d<R$): $B\propto d$ (Linear increase). $B=\frac{{\mu }_{0}Id}{2\pi R^2}$.At the surface ($d=R$): $B$ is maximum.Outside the conductor ($d>R$): $B\propto \frac{1}{d}$ (Hyperbolic decrease). $B=\frac{{\mu }_{0}I}{2\pi d}$.The graph shows a straight line from the origin to $R$, followed by a curve decreasing to zero.