Final answer:
HCl is the gas that is least likely to behave ideally among the given options.
Step-by-step explanation:
In order to determine which gas is least likely to behave ideally, we need to consider factors such as size, shape, and intermolecular forces. Typically, larger and more polar molecules are less likely to behave ideally. Among the given options, HCl (option c) is least likely to behave ideally. HCl is a polar molecule with strong dipole-dipole forces, which deviate from the assumptions of an ideal gas.