The correct option is A 6235.5 J
For O2 (diatomic) molecule:
Translational degree of freedom = 3,
and rotational degree of freedom = 2, so
Translational energy, ET=32RT,Rotational energy, ER=RT
Here, Temperature T=300 K, R=8.314 J mol−1 K−1
Etotal=32RT+RT=52RT=52×8.314×300=6235.5 J