The diagram shows a slab of insulating material that is infinitely long in the $x$- and $z$-directions.
The slab's thickness in the $y$-direction is $2D$. The slab carries a uniform charge density $\rho$ distributed throughout its volume. Point $P$ is at $y = d$ which is inside the slab material $(d < D)$.
Which of the following expresses the electric field strength at point $P$?