Oxygen gas is paramagnetic. This observation is best explained by:

The hybridization of the molecular orbital description of  ${\mathrm{O}}_2$ is the best explanation for the paramagnetic nature of O2.

According to valence bond theory, bonds are created by atomic orbital; electrons are paired at the time of overlapping, making oxygen a diamagnetic species. However, according to experiment, oxygen molecules have a paramagnetic property, which molecular orbital theory confirms.

Hence, the correct option is C.