A square surface of side $L\text{ meter}$ in the plane of the paper is placed in a uniform electric field $E\text{ (volt/m)}$ acting along the same plane at an angle $\theta$ with the horizontal side of the square as shown in figure. The electric flux linked to the surface, in units of volt-m, is:

Electric flux is defined as $\varphi =\vec{E}\cdot \vec{A}=EA\cos {\varphi }_{angle}$, where ${\varphi }_{angle}$ is the angle between the electric field and the area vector.
The area vector $\vec{A}$ is always perpendicular to the surface.
Since the square and the electric field are both in the plane of the paper, the field lines are parallel to the surface, meaning the angle between $\vec{E}$ and $\vec{A}$ is $90^{\circ }$.
$$\varphi =EA\cos (90^{\circ })=0$$