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.
