a. Due to the non-polar nature of CO2 molecules, London dispersion forces are the only intermolecular forces that may interact with them.
b. Due to the presence of a nitrogen atom with a single pair of electrons, NH3 molecules are polar and are capable of both hydrogen bonding and dipole-dipole interactions.
c. Due to their polar nature, HCl molecules are capable of both hydrogen bonding and dipole-dipole interactions.
d. Since C6H6 (benzene) molecules are non-polar, London dispersion forces are the only intermolecular forces that can act between them.