The correct choice is (c) Raoult’s law
To explain I would say: Raoult’s law quantifies the relative lowering in vapor pressure. From the law, it follows that p1 = X1 x p^01. If p^01 was the original pressure before X2 mole fraction of solute was added to the solvent then reduction in vapor pressure is given as:
Δp1 = p^01 – p1
= p^01 – p^01 X1
= p^01(1 – X1)
= p^01X2
Therefore, the relative reduction in vapor pressure (∆p1/p^01) = X2 i.e. mole fraction of solute in the solution.