Polar Molecules
Molecular polarity occurs when there is an uneven distribution of charge throughout a molecule. This occurs when one region is much more electronegative than an opposite region, causing there to be an uneven distribution of electrons in the molecule.
This is seen in water, as oxygen is much more electronegative than hydrogen. The oxygen end of a water molecule has a partial negative charge whereas the hydrogen atoms have a partial positive charge.
This is also seen in ionic compounds, such as HCl. Ionic bonds are the most polar type of bond, as there is an exchange of electrons.
However, many molecules are not polar. One example is diatomic molecules (consider HOBrFNCl). Each atom in a molecule has equal electronegativity, so the pull is equal on both sides. These molecules have no polarity.
Polar: a,c
Nonpolar: b,d