Two Zener diodes (A and B) having breakdown voltages of 6 V and 4 V respectively, are connected as shown in the circuit below. The output voltage  V0 variation with input voltage linearly increasing with time, is given by (Vinput=0V at t = 0)( figures are qualitative)

To get an answer from the options there must be some r as shown   Initially, input voltage is zero, Thus, output voltage is also zero. So, graph starts from origin.As input voltage increases linearly with time, output voltage also increases linearly with time.A moment comes when 4V Zener diode breaks and some current starts flowing through 100 ohm resistor. As a result, the rate of increase of output voltage decreases.Further, when 6V Zener diode also breaks, then a constant output voltage of 6V is obtained, irrespective of increase in input voltage.