Correct Answer - B

(b) KEY CONCEPT: The time required for constructive interference equal to the time period of a wave pulse.

For a string frequency `f = (1)/(2l) sqrt((F)/(m))`

:. Time period, `T = 2lsqrt((m)/(F))`

`F = 1.6N`, `m = (mass)/(length) = (10^(-2))/(0.4) = 2.5 xx 10^(-2)`

:. `T = 2 xx 0.4 sqrt((2.5 xx 10^(-2))/(1.6)) = 0.1 sec`.