The time constant of and R- C cirucit during charging is them time in which the charge on the condenser plates, as compared to maximum charge `(q_0)` becomes `(q//q_0)xx `100,` which is equal to …….. .

Explanation

`q = q_(0)(1 - e^(-t/tau))`. Putting `t = tau,` we get
`(q)/(q_(0)) = 1-e^(-1)`
or `(q)/(q_(0))xx100 = (e-1)/(e ) xx100 = 63.2%`