How many atoms will be bisected by the plane of symmetry in the given molecule of $1,3$-dichlorocyclobutane (trans-isomer)?

(C) The molecule is $trans-1,3$-dichlorocyclobutane.
It possesses a plane of symmetry that passes through the two carbon atoms bearing the chlorine substituents ($C_1$ and $C_3$) and the two chlorine atoms themselves.
This plane bisects the $C_1$ and $C_3$ atoms,the two $Cl$ atoms attached to them,and the two $H$ atoms attached to $C_1$ and $C_3$.
Total atoms bisected = $2$ $(C)$ + $2$ $(Cl)$ + $2$ $(H)$ = $6$ atoms.