In which of the below compounds are the bond(s) most polar ?

Polarity deals with the difference in electronegativity between the two bonded atoms. Fluorine with an electronegativity of 4.0 is the most electronegative substance. Hydrogen has an electronegativity of 2.1, and so the difference between the two is 4.0-2.1= 1.9 giving us the highest polarity of the choices offered, so b.