Q.

In pure methanol, molecules are hydrogen bonded. on adding acetone, its molecules come in between the host molecules and break some of the hydrogen bonds between them. Therefore, the inter molecular attractive forces between the solute-solvent molecules are weaker than those of between the solute-solute and solvent-solvent molecules. On the other hand, other three remaining options will show negative deviation from Raoult's law where the inter molecular attractive forces between the solute-solvent molecules are stronger than those in between the solute-solute and solvent-solvent molecules.