The correct option is C 2.5 ms
Let the current be, i=i0sinωt.
Also, let t1 and t2 be the time instants for consecutive appearances of the maximum value and the rms value of the current, respectively.
⇒ωt2=3π4⇒t2=3π4ω [As we are taking the rms value after the first peak value]
So, the time taken by the current to change from its first maximum value to the consecutive rms value is,
⇒t2−t1=π4(2πf)=18×50=2.5×10−3 s=2.5 ms