Calculate the volume occupied by 200 $g$ of $S{O}_3$ gas at $STP$.

The correct option is C $56L$
Volume occupied by 80 $g$ of $S{O}_3$ at $STP$ = 22.4 $L$

Thus the volume occupied by 200 $g$ of $S{O}_3$ = $\frac{22.4\times 200}{80}$ = 56 $L$

Hence, the correct option is $\text{C}$