IR active molecules are those that exhibit infrared absorption in the infrared spectroscopy technique. Infrared spectroscopy measures the vibrations and rotations of molecules, and IR active molecules have certain molecular motions that can absorb infrared radiation.

The correct answer to the given question is option 'A,B,C', which means that CO2, CS2, and OCS are IR active molecules. Let's understand why these molecules are IR active.

1. CO2 (Carbon Dioxide):
- Carbon dioxide (CO2) is a linear molecule consisting of one carbon atom and two oxygen atoms.
- Infrared spectroscopy measures the stretching and bending vibrations of molecules.
- CO2 has three vibrational modes: symmetric stretching, asymmetric stretching, and bending.
- The symmetric stretching mode is IR active, which means it absorbs infrared radiation and produces a characteristic peak in the IR spectrum.
- This is because the symmetric stretching motion causes a change in the dipole moment of the molecule, resulting in the absorption of infrared radiation.

2. CS2 (Carbon Disulfide):
- Carbon disulfide (CS2) is a linear molecule consisting of one carbon atom and two sulfur atoms.
- CS2 has three vibrational modes: symmetric stretching, asymmetric stretching, and bending.
- Both the symmetric and asymmetric stretching modes are IR active, which means they absorb infrared radiation and produce characteristic peaks in the IR spectrum.
- This is because the stretching motions cause a change in the dipole moment of the molecule, resulting in the absorption of infrared radiation.

3. OCS (Carbonyl Sulfide):
- Carbonyl sulfide (OCS) is a linear molecule consisting of one carbon atom, one oxygen atom, and one sulfur atom.
- OCS has three vibrational modes: symmetric stretching, asymmetric stretching, and bending.
- Both the symmetric and asymmetric stretching modes are IR active, which means they absorb infrared radiation and produce characteristic peaks in the IR spectrum.
- This is because the stretching motions cause a change in the dipole moment of the molecule, resulting in the absorption of infrared radiation.

In conclusion, CO2, CS2, and OCS are IR active molecules because they have vibrational modes that cause a change in their dipole moments, leading to the absorption of infrared radiation.