Explanation:
Infrared (IR) spectroscopy and Raman spectroscopy are two different techniques used to study the vibrations of molecules. IR spectroscopy involves the absorption of infrared radiation by a molecule, leading to changes in the vibrational energy levels of the molecule. Raman spectroscopy involves the scattering of light by a molecule, leading to changes in the vibrational energy levels of the molecule.

IR-Active and Raman-Active Molecules:
In general, a molecule that is IR-active will also be Raman-active, and vice versa. This is because both techniques involve changes in the vibrational energy levels of the molecule. However, there are some exceptions to this rule.

IR-inactive and Raman-Active Molecules:
One such exception is nitrogen gas (N2). Nitrogen gas is a homonuclear diatomic molecule, which means that it consists of two nitrogen atoms bonded together. Nitrogen gas has no net dipole moment, which means that it does not absorb infrared radiation. Therefore, nitrogen gas is IR-inactive. However, nitrogen gas does have a Raman-active vibration, which is the symmetric stretching vibration of the nitrogen-nitrogen bond. This vibration leads to changes in the polarizability of the molecule, which can be detected by Raman spectroscopy.

Other Options:
HCl is a polar molecule and has a dipole moment. Hence it is both IR and Raman active.
SO2 is a polar molecule and has a dipole moment. Hence it is both IR and Raman active.
Proteins are complex molecules and have many vibrational modes. They are both IR and Raman active.

Conclusion:
In conclusion, the molecule which is IR-inactive but Raman-active is nitrogen gas (N2).