A capacitor of 4mF is connected to a 15 V supply through 1 mega ohm resistance. The time taken by the capacitor to charge upto 63.2 % of its final charge will be

2s  3s 4s 5s

4s

$ q = 0.632 q_0 = q_0 e^{-\frac{t}{RC}} $ $ 0.632 = e^{-\frac{t}{RC}} $ $ t = RC = 4s$