The correct option is A 5 s
Given:
Wavelength of the sound, λ=40 cm =0.4 m
Time period, T=0.0005 s
Distance covered, s=4 km=4000 m
Speed of the sound wave is given by:
v=fλ
But frequency is inverse of the time period.
f=1T
⟹v=λT
v=0.4 m0.0005 s
v=800 m/s
Using formula for speed, time taken to cover 4 km will be:
t=sv
t=4000 m800 m/s
t=5 s