Till input voltage Reaches $4V$ No zener is in Breakdown Region So $V _{0}= V _{ i }$ Then Now when $V _{ i }$ changes between $4 V$ to $6 V$ One Zener with $4 V$ will Breakdown are $P.D$. across This zener will become constant and Remaining Potential will drop. acro

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 $V _{0}$ and $t.$ will be

We have to Assume some resistance in series with source.