Dipole-dipole attractions are strongest when the molecules have a large dipole moment. The dipole moment of a molecule is a measure of the separation of positive and negative charges within the molecule. The larger the dipole moment, the stronger the dipole-dipole attractions.

Of the given compounds, CH3Cl, HCN, and CH3COOH have the largest dipole moments.

- CH3Cl has a dipole moment of 1.86 D. The electronegative Cl atom pulls electrons away from the C atom, creating a partial positive charge on the C atom and a partial negative charge on the Cl atom.

- HCN has a dipole moment of 2.98 D. The electronegative N atom pulls electrons away from the H atom, creating a partial positive charge on the H atom and a partial negative charge on the N atom.

- CH3COOH has a dipole moment of 1.70 D. The electronegative O atom in the -COOH group pulls electrons away from the C atom, creating a partial positive charge on the C atom and a partial negative charge on the O atom.

Therefore, dipole-dipole attractions are most important intermolecular force in CH3Cl, HCN, and CH3COOH.