The number of non-polar molecules from the following is HF, H _ 2...

The molecules which have perfect geometry and no lone pairs are zero dipole moment molecules. Those molecules termed as non-polar molecules. Amongst the given compounds, ${CH}_{4}, {CO}_{2}, {H}_{2}, {BF}_{3}$ are non polar molecules.

${H}_{2}O$ has two lone pairs and two bond pairs. ${SO}_{2}$ has two sigma bond pairs and one lone pair. ${CHCl}_{3}$ has net dipole moment because the bond moments do not cancel with each other. ${NH}_{3}$ have one lone pair and three bond pairs. In $HF$ bond moment is the dipole moment. Hence, all these molecules have some net dipole moment.

The non-polar molecules are ${CO}_{2}, {H}_{2}, {CH}_{4}$ and ${BF}_{3}$