Assertion (A): Energy of resonance hybrid is equal to the average of energies of all canonical forms.
Reason (R): Resonance hybrid cannot be presented by a single structure.
A is not correct but R is correct.
Analysing the assertion :
Resonance hybrid is the real structure of a molecule which has the contribution of all canonical structures. It is not equal to the average of all canonical structures. More stable canonical structure has more contribution towards resonance hybrid and less stable canonical structure has less contribution towards resonance hybrid.
So, the assertion is wrong.
Analysing the reason :
Resonance hybrids cannot be presented by a single structure. It involves contribution from all canonical structures.
So, the reason is correct.
Hence, assertion is not correct but reason is correct.
So, option (D) is the correct answer.