Till input voltage reaches 4 V. No zener is in breakdown region so V0=Vi. Then now when Vi changes between 4V to 6V one zener with 4V will breakdown and P.D. across this zener will become constant and remaining potential will dorp across resistance in series with 4 V zener. Now current in circuit increases abruptly and source must have an internal resistance due to which some potential will get drop across the source also so correct graph between V0 and t will be