a.) SbH3 > AsH3 > PH3 > NH3 . The electronegativity of the X atom in this XH3 series determines the polarity of the molecule. Electronegativity increases as you move up a column in the periodic table, therefore the X-H bonds are more polarized as you move up the periodic table.
b.) Cl2 > H2O > HF. There are two factors here- the geometry of the molecule and the electronegativity difference between the atoms. Since Cl is bonded to itself there is no polarity. H2O is polar but the H-O bond is less polar than the H-F bond. Since F is more electronegative than O, the H-F is the most polar.
c.) Cl2 > AlCl3 > PCl3 > KCl. Like the previous example the Cl-Cl bond is not polarized because it contains two Cl atoms with the same electronegativity. KCl is very polar since K is way to the left of the periodic table and electronegativty increases as you move right in the periodic table. Al is to the left of P in the periodic table, so the P-Cl bond is more polar, and thus PCl3 is more polar than AlCl3.