Chapter 21 - AC Circuits - Learning Path Questions and Exercises - Multiple Choice Questions - Page 747: 5

(b) $\frac{1}{120}s$

AC frequency in US = 60Hz Time for 1 cycle = 1/60 s Number of times power reaches maximum in 1 cycle = 2 times, So time elapsed between maximum successive power = $\frac{1}{60}\times\frac{1}{2}=\frac{1}{120}s$