Answer
$J = 400~A/m^2$
Work Step by Step
The current density is: $~~J = \frac{current}{area} = \frac{i}{\pi~r^2}$
Since the graph of $i$ versus $r^2$ is a straight line, then the current density is uniform.
We can find the current density:
$J = \frac{i}{\pi~r^2}$
$J = \frac{5.0\times 10^{-3}~A}{(\pi)~(4.0~\times 10^{-6}~m^2)}$
$J = 400~A/m^2$
