The number of molecules with zero net dipole moment among BF3, CO2, XeF4, SF6, NF3, PCl5, cis-1,2-dichloroethene and CCl4 is

A molecule is non-polar when its bond dipoles cancel by symmetry. $\mathrm{BF_3}$ (trigonal planar), $\mathrm{CO_2}$ (linear), $\mathrm{XeF_4}$ (square planar), $\mathrm{SF_6}$ (octahedral), $\mathrm{PCl_5}$ (trigonal bipyramidal) and $\mathrm{CCl_4}$ (tetrahedral) are all symmetric with zero dipole moment. $\mathrm{NF_3}$ (pyramidal, lone pair) is polar, and cis-1,2-dichloroethene has both C-Cl dipoles on the same side, so it is polar (the trans isomer is non-polar). Total $=6$.