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

The correct option is acetonitrile and acetone.

To understand why this is the correct answer, let us analyze the types of intermolecular forces present in each of the given mixtures:

1. Benzene and ethanol: Benzene ($C_6{H}_6$) is a non-polar molecule, while ethanol ($C_2{H}_5\text{OH}$) is a polar molecule capable of hydrogen bonding. The major interaction between a polar and a non-polar molecule is dipole-induced dipole interaction.
2. Acetonitrile and acetone: Both acetonitrile (${\mathrm{CH}}_3\text{CN}$) and acetone (${\mathrm{(CH}}_3{)}_2\text{CO}$) are polar molecules. Acetonitrile has a strong dipole moment due to the polar cyano group ($-\text{C}\equiv \text{N}$), and acetone has a strong dipole moment due to the polar carbonyl group ($>\text{C}=\text{O}$). Since both components are polar, the major intermolecular interaction between them is the dipole-dipole interaction.
3. KCl and water: Potassium chloride ($\text{KCl}$) is an ionic compound that dissociates into ions (${\text{K}}^{+}$ and ${\text{Cl}}^{-}$) in water, which is a polar solvent. The major interaction here is the ion-dipole interaction.
4. Benzene and carbon tetrachloride: Both benzene ($C_6{H}_6$) and carbon tetrachloride (${\mathrm{CCl}}_4$) are non-polar molecules. The major interaction between non-polar molecules is dispersion forces (London forces).

Therefore, the mixture of acetonitrile and acetone exhibits dipole-dipole interaction as its major intermolecular force.