The correct option of this question will be (d).

**Solution —**

Let two long parallel thin wires $X$ and $Y$ carry current $I$ and separated by a distance $b$ apart.

The magnitude of magnelic field $B$ at any point on $Y$ due to current $I_{1}$ in $X$ is given by

$$B=\frac{\mu_{0}}{2 \pi} \frac{I_{1}}{b}$$

The magnitude of force acting on length $I$ of $Y$ is

$$F=I_{2} B l=I_{2}\left(\frac{\mu_{0}}{2 \pi} \frac{I_{1}}{b}\right) l$$

Force per unit length is

$$\frac{F}{l}=\frac{\mu_{0}}{2 \pi} \frac{I_{1} I_{2}}{b}$$

Given $I_{1}=I_{2}=I$,

therefore

$$\frac{F}{l}=\frac{\mu_{0}}{2 \pi} \frac{I^{2}}{b}$$