CH3Cl is a non polar molecule because the electronegativity of carbon and chlorine is almost the same. So, there is no significant difference in electronegativity between carbon and chlorine atoms. Therefore, the molecule is non polar.

H2O is a polar molecule because the electronegativity of oxygen is more than hydrogen. So, there is a significant difference in electronegativity between oxygen and hydrogen atoms. Therefore, the molecule is polar.

OF2 is a polar molecule because the electronegativity of oxygen is more than fluorine. So, there is a significant difference in electronegativity between oxygen and fluorine atoms. Therefore, the molecule is polar.

BeCl2 is a non polar molecule because the electronegativity of beryllium and chlorine is almost the same. So, there is no significant difference in electronegativity between beryllium and chlorine atoms. Therefore, the molecule is non polar.

NH3 is a polar molecule because the electronegativity of nitrogen is more than hydrogen. So, there is a significant difference in electronegativity between nitrogen and hydrogen atoms. Therefore, the molecule is polar.