Answer
We can rank the arrangements according to the magnitude of the net force on the central wire due to the currents in the other wires:
$(b) \gt (d) \gt (c) \gt (a)$
Work Step by Step
We can write an expression for the force on a current-carrying wire due to another current-carrying wire:
$F = \frac{\mu_0~L~i_1~i_2}{2\pi~d}$
According to the text on page 854: "Parallel wires carrying currents in the same direction attract each other, whereas parallel wires carrying currents in opposite directions repel each other."
We can consider the net force on the central wire:
(a) By symmetry, the forces due to the other four wires cancel out.
$F_{net} = 0$
(b) All four forces on the central wire are directed to the right. $F_{net}$ is a maximum.
(c) The forces on the central wire due to the two inner wires are directed to the right.
The forces on the central wire due to the two outer wires are directed to the left.
The net force is directed to the right but it is not a maximum.
(d) The forces on the central wire due to the two inner wires are directed to the right.
The forces on the central wire due to the two outer wires cancel out.
The net force is directed to the right but it is not a maximum.
Note that the net force in arrangement (c) is less than the net force in arrangement (d) because the component of the force directed to the left in arrangement (c) decreases the net force on the central wire.
We can rank the arrangements according to the magnitude of the net force on the central wire due to the currents in the other wires:
$(b) \gt (d) \gt (c) \gt (a)$
