There is a square wire frame ABCD of side equal to b and whose centre is at a distance $\mathrm{OP}=\frac{\mathrm{b}}{2}$  from an infinite line charge (parallel to BC and AD) with linear charge density $\lambda$  then [plane of the frame is perpendicular to line OP]

Imagine a cube of side b , with infinite line charge passing through its center.

Since, electric field is parallel to top and bottom faces, flux passing through them will be zero.

$\begin{array}{l}{\varphi }_{cube}=\frac{q_{enclosed}}{{\epsilon }_0}=\frac{\lambda b}{{\epsilon }_0}\\ \therefore {\varphi }_{eachface}=\frac{\lambda b}{4{\epsilon }_0}\end{array}$

All points on line BC are equidistant from line charge , hence electric field is constant.