If the straight line $y=mx+c$ is parallel to the axis of the parabola $y^2=lx$ and intersects the parabola at $\bigg ( \large\frac{c^2}{8}$$,c\bigg)$ then the length of the latus rectum is

Question

$$\begin {array} {1 1} (a)\;2 & \quad (b)\;3 \\ (c)\;4 & \quad (d)\;8 \end {array}$$