Liquid oxygen is blue in colour due to :

Liquid oxygen is blue in colour due to electron transition from HOMO to LUMO in visible region. Liquid oxygen is strongly paramagnetic in nature because it has two unpaired electrons in it. Due to the magnetic imbalance caused by the two unpaired electrons, red light is absorbed in the visible spectrum. The typical hue of liquid oxygen changes to a pale blue tint as a result of the absorption of red.

The transition between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) in the ground state has the lowest energy. An electron is stimulated to the LUMO and enters an excited state as a result of the EM radiation being absorbed.

Hence, the option A is correct.