. "B is the best choice - (Appositive) -- Emphasis is on the ERA, when Y was Mayor.A - incorrect - Cannot be past and past-perfect at the same time - the city has transformedC - incorrect - use of it - ambiguousD, E - incorrect - use of during is wrong in D and E because there is no antecedent for during which .We need 'an era' as an appositive otherwise the clauses at the end would modify the" . . .