That benzene's bonding is characterized by delocalized pi electrons rather than fixed alternating single and double Kekule structures is a standard textbook fact, rendering a citation unnecessary.
The claim that the Kekule structure is insufficient due to electron delocalization is a foundational and uncontroversial fact of organic chemistry (resonance/delocalization theory). Under the instructions, trivially true or standard foundational facts of this nature are classified as COMMON_KNOWLEDGE, meaning no citation is strictly needed, though relevant papers discussing molecular orbitals and benzene's electronic structure are appropriately tagged under the for_indices.