The figure  shows an experimental plot for discharging of a capacitor in an R-C circuit. The time constant  of this circuit lies between :

150 sec and 200 sec
0 and 50 sec
50 sec and 100 sec
100 sec and 150 sec
                
                  
R C Circuit Voltage -
- wherein

During discharging of a capacitor
Where   is the time constant of RC circuit
This voltage occurs at time lies between 100 sec and 150 sec . Hence, time constant   of this circuit lies between
100 sec and 150 sec