An aqueous solution of the disodium salt shown is electrolysed. Which hydrocarbon is liberated at the anode?

The compound shown is disodium butanedioate (succinate). Each carboxylate group is lost as $\mathrm{CO}{}_2$ at the anode, leaving a diradical $\cdot \mathrm{CH}{}_2\mathrm{CH}{}_2\cdot$ that collapses to ethene. Dimerisation to butane or ring closure to cyclobutane does not occur, and ethane would need two extra hydrogens.