Negative deviation refers to the situation when the observed behavior of a mixture deviates from the ideal behavior in a way that the intermolecular forces between the components are weaker than expected. This typically occurs when the components of the mixture have different molecular sizes and polarities. Among the given options, the pair of substances that shows negative deviation is chloroform and benzene.

Explanation:

Intermolecular forces: Chloroform (CHCl3) is a polar molecule due to the electronegativity difference between carbon and chlorine atoms. It exhibits dipole-dipole interactions between its molecules. On the other hand, benzene (C6H6) is a nonpolar molecule with a symmetrical hexagonal structure, and it experiences weak London dispersion forces.

Size difference: Chloroform has a smaller molecular size compared to benzene.

Key points:
- Chloroform is a polar molecule, while benzene is nonpolar.
- Chloroform has a smaller molecular size compared to benzene.

Behavior of the mixture:
When chloroform and benzene are mixed together, the polar chloroform molecules can interact with the nonpolar benzene molecules through dipole-induced dipole interactions. This interaction weakens the intermolecular forces between the chloroform molecules, resulting in negative deviation from Raoult's law.

Explanation:
- Chloroform molecules interact with benzene molecules through dipole-induced dipole interactions.
- These interactions weaken the intermolecular forces between chloroform molecules.
- As a result, the observed vapor pressure of the mixture is lower than predicted by Raoult's law for an ideal solution.
- Negative deviation indicates weaker intermolecular forces between the components compared to the ideal case.

Other options:
a) Carbon tetrachloride and chloroform: Both carbon tetrachloride (CCl4) and chloroform (CHCl3) are polar molecules. Their molecular sizes are relatively similar. When mixed, they show positive deviation from Raoult's law.
c) Acetone and benzene: Acetone (CH3COCH3) is a polar molecule, while benzene (C6H6) is nonpolar. Due to the difference in polarities and molecular sizes, they exhibit positive deviation from Raoult's law.
d) Methyl alcohol and water: Both methyl alcohol (CH3OH) and water (H2O) are polar molecules. When mixed, they form strong hydrogen bonding, resulting in positive deviation from Raoult's law.

In summary, among the given options, chloroform and benzene show negative deviation from Raoult's law due to the weaker intermolecular forces between their components.