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`.