= Solution
Event a probes <electron transfer> between the semiquinone and dianion. Benzoquinone's smaller framework localizes charge and undergoes a greater change in bonding and solvation on this reduction; a larger effective reorganization barrier can make its heterogeneous transfer insufficiently fast for Nernst equilibrium on the $50\,\mathrm{mV\,s^{-1}}$ time scale. Anthraquinone's extended framework distributes the added charge and can support the more reversible response seen here.
This is a kinetic interpretation of quasi-reversibility, not a claim that benzoquinone cannot be reduced. Follow-up chemistry or <adsorption> can also distort peak separation and return-current ratios; scan-rate-dependent measurements would distinguish them from slow <electron transfer>. A single trace does not uniquely identify the cause.
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