At a given temperature
a) Vapour pressure of a solution containing nonvolatile solute is proportional to mole fraction of solvent
b) Lowering of vapour pressure of solution containing nonvolatile solute is proportional to mole fraction of solute
c) Relative lowering of vapour pressure is equal to mole fraction of solute The correct combination is:

A nonvolatile solute solution's vapour pressure is inversely correlated with the mole fraction of the solvent. According to the mole fraction of the solution, the vapour pressure of solutions containing nonvolatile solutes decreases. The mole fraction of the solute is equal to the relative decrease in vapour pressure.

So, option C) a, b, and c alone constitute the necessary solution.