Which one of the following is paramagnetic in nature?

1. Paramagnetism is a property of materials that are weakly attracted to an external magnetic field. This occurs due to the presence of unpaired electrons in the material's atoms or molecules. 2. Iron is ferromagnetic (strongly attracted to magnetic fields). 3. Diatomic Hydrogen (H2) and diatomic Nitrogen (N2) molecules have all their electrons paired in molecular orbitals, making them diamagnetic (weakly repelled by magnetic fields). 4. The diatomic Oxygen (O2) molecule, according to molecular orbital theory, has two unpaired electrons in its highest occupied molecular orbitals (antibonding π* orbitals). This makes Oxygen paramagnetic.