Which one of the following is paramagnetic in nature?

Oxygen is the correct answer because of its unique electronic structure. While many gases are diamagnetic, oxygen molecules have two unpaired electrons in their outer shells. According to Molecular Orbital Theory, these unpaired electrons align with an external magnetic field, which makes the substance paramagnetic. Iron is ferromagnetic, meaning it is strongly attracted to magnets. Hydrogen and nitrogen are diamagnetic because all of their electrons are paired, causing them to be weakly repelled by magnetic fields. Therefore, among the given options, only oxygen exhibits paramagnetism.