Which of the following explains how polar molecules form?

a. electrons are more attracted to one of the atoms involved in a bond.
b. the two atoms in a bond share electrons equally.
c. one atom transfers its electrons to another atom.
d. protons are more attracted to one of the atoms involved in a bond.

The answer is A. It'ss based on electronegativity (not sure if this is covered before A level) but essentially in the covalent bond, the bonding electron pairs are more attracted towards one of the atoms than the other. This creates a slight difference in charge.
So for instance in water, the oxygen is greedy and pulls the bonding electron pair away from hydrogen, closer to itself. This gives the oxygen molecule a slightly negative charge and the hydrogens a slight positive charge. 

What subject are you on