Which of the following aqueous samples has the lowest vapor pressure above the pure liquid?

Which of the following aqueous samples has the lowest vapor pressure above the pure liquid? (For ionizing solutes, assume complete ionization, with no ion-pairing.) 

a)  0.4 m NH4NO3                      b)  0.5 m Na2SO4                 c)  0.2 m Ca(NO3)2

d)  0.3 m KCl AND 0.2 m Na2SO4                   e)  pure water

I know that for V.P. the solution with the greatest number of disruption sites will have the lowest V.P. I don't understand how to find the ions/formula unit. Please explain how to find thay using the examples above.