A capacitor in a single-loop RC circuit is charged to 85% of its final potential difference in 2.4 s. What is the time constant for this circuit

The  time constant is  

Explanation:

From the question we are told that

     the time take to charge is  

The mathematically representation for voltage potential of a capacitor at different time is

Where    is the time constant  

            is the potential of the capacitor when it is full

     So  the capacitor potential will be  100%  when it is full thus  100%  =  1  

and from the  question we are told that the  at the given time the potential of the capacitor is 85% = 0.85 of its final potential so

      V  = 0.85

Hence

think its the third option