Answer
We can rank the wires according to the magnitude of the electric field at the center:
$a = c \gt b$
Work Step by Step
We can consider the equation: $~~E = \rho~J$
Since the wires are made from the same material, then $\rho$ is equal for all three wires.
Therefore, the magnitude of the electric field is greater when the current density $J$ is greater.
At $r = 0$, wire a and wire c have the same current density $J$, while $J=0$ in wire b.
We can rank the wires according to the magnitude of the electric field at the center:
$a = c \gt b$
