Among the following mixtures, dipole-dipole as the major interaction, is present in

The type of intermolecular force that dominates in any mixture depends on the nature of the species present.
• Non-polar molecules interact mainly through London (dispersion) forces.
• Polar molecules (with a permanent dipole moment) attract each other through dipole-dipole interactions.
• When ions are present, the strongest force is almost always ion-dipole (between an ion and a polar solvent).
• Hydrogen bonding is only possible when the molecules contain N-H, O-H or F-H bonds and is stronger than ordinary dipole-dipole forces.

Now analyse each option one by one.

Option A (benzene + ethanol)
Benzene, $$C_6H_6$$, is non-polar whereas ethanol, $$CH_3CH_2OH$$, is polar and even forms intermolecular hydrogen bonds with itself.
The interaction between benzene and ethanol is therefore dipole-induced-dipole (the dipole of ethanol polarises benzene). Dipole-dipole is not the major interaction.

Option B (acetonitrile + acetone)
Acetonitrile, $$CH_3CN$$, possesses a strong $$C \equiv N$$ dipole, and acetone, $$CH_3COCH_3$$, possesses a strong carbonyl $$C=O$$ dipole. Neither molecule can form hydrogen bonds with itself or with each other because they lack N-H, O-H or F-H bonds. As both partners are polar but non-ionic, the dominant attraction between them is ordinary dipole-dipole interaction.

Option C (KCl + water)
Potassium chloride furnishes $$K^+$$ and $$Cl^-$$ ions in solution. The predominant force between these ions and polar water molecules is ion-dipole, which is stronger than simple dipole-dipole.

Option D (benzene + carbon tetrachloride)
Both benzene and carbon tetrachloride, $$CCl_4$$, are non-polar. Their only attractive force is London dispersion.

Hence, the only mixture in which dipole-dipole interaction is the major attractive force is Option B.

Option B which is: acetonitrile and acetone