Answer
We can rank the wires according to the value of the current:
$(a) = (c) \gt (b) = (d)$
Work Step by Step
$\int~B\cdot ds = \mu_0~i_{enc}$
Therefore:
$B = \frac{\mu_0~i_{enc}}{2\pi~r}~~~$ where $r$ is the radial distance from the center of the wire.
At large values of $r$ on the graph, $a$ and $c$ have greater magnetic fields than $b$ and $d$
Therefore, the enclosed current in curves $a$ and $c$ must be greater than the enclosed current in curves $b$ and $d$
We can rank the wires according to the value of the current:
$(a) = (c) \gt (b) = (d)$
