The shape of sulphate ion is

Which of the following molecules is nonpolar?

$\left(\mathrm{i}\right)$ ${\mathrm{PbCl}}_4$
$\left(\mathrm{ii}\right)$ ${\mathrm{BF}}_3$
$\left(\mathrm{iii}\right)$ ${\mathrm{SnCl}}_2$
$\left(\mathrm{iv}\right)$ ${\mathrm{CS}}_2$

If the $\mathrm{z}$-axis is the molecular axis, then $\mathrm{\pi }$ $\mathrm{MOs}$ are formed by the overlap of-

$\left(\mathrm{i}\right)$ ${\mathrm{p}}_{\mathrm{z}}$ and ${\mathrm{p}}_{\mathrm{z}}$
$\left(\mathrm{ii}\right)$ ${\mathrm{p}}_{\mathrm{y}}$ and ${\mathrm{p}}_{\mathrm{y}}$
$\left(\mathrm{iii}\right)$ ${\mathrm{s}}_{\mathrm{z}}$ and ${\mathrm{p}}_{\mathrm{z}}$
$\left(\mathrm{iv}\right)$ ${\mathrm{p}}_{\mathrm{x}}$ and ${\mathrm{p}}_{\mathrm{x}}$