Explanation

(a) The force per unit length between two parallel wires carrying currents $i_{1}and\ i_{2}$ separated by a distance $R$ is given by
$\frac{F}{l} = \frac{\mu_{0}}{2\pi}\frac{i_{1}i_{2}}{2}$
$\Rightarrow \frac{F}{l} \propto \frac{1}{R}$
Hence, graph between force per unit length and distance between wires is a straight line.