Concept:Complete hydrogenation adds hydrogen across all double bonds of benzene, producing a saturated ring compound.Explanation:Benzene has the formula C6H6.It contains a ring with alternate double bonds, but the structure is stabilised by delocalisation.On complete hydrogenation with hydrogen gas and a nickel catalyst, all unsaturation is removed.Each carbon atom gains one hydrogen atom, so the product has the formula C6H12.This saturated cyclic compound is cyclohexane.Answer:Cyclohexane