The magnetic moment of the O3 molecule is zero because it is diamagnetic. Despite having unpaired electrons in its molecular orbitals, the overall molecular configuration leads to no net magnetic moment. This is due to the symmetric distribution of electrons in the molecule.
Since the O3 molecule has no unpaired electrons in its ground state and no net magnetic moment, the correct answer is 0 Bohr magnetons.
Resonance in X$_2$Y can be represented as
The enthalpy of formation of X$_2$Y is 80 kJ mol$^{-1}$, and the magnitude of resonance energy of X$_2$Y is: