Answer
We can rank the wires according to the magnitude of the magnetic field on the surface of the wire:
$(a) \gt (c) \gt (b) \gt (d)$
Work Step by Step
Each curve changes from increasing to decreasing at the point where $r$ is the radius of each wire.
The maximum point of each curve is the magnitude of the magnetic field on the surface of the wire.
We can rank the wires according to the magnitude of the magnetic field on the surface of the wire:
$(a) \gt (c) \gt (b) \gt (d)$
