Without solving for the dipole moment, we can easily determine which among the common gases has the smallest dipole moment just by determining the differences in their electronegativity. The greater the difference in the electronegativity, the higher is the value of the dipole moment.
From the given above, there are obvious differences between the electronegativity between the atoms composing LiF, ClF, and HF. For Cl2, since this is the same molecule then, the difference in the electronegativity is zero.
Answer: Cl2.
