Correct Answer - C
Vapour pressire of a solvent is lowered by the presence of solute in it. Lowering in vapour pressure is a colligative property i.e., it depends on the no. of particles present in the solution. `Cu(NO_(3))_(2)` give the maximum no. of ions. (i.e., 3) so it causes the greatest lowering in vapour pressure of water.
