The correct option is C 1412 Hz
In this case, velocity of source Vs=18 kmph=18×518=5 m/s
and similarly,
Velocity of observer Vo=36 kmph=10 m/s
Let N0 be the frequency of sound heard by the observer, if both the source of sound and observer are at rest.
Let v be the velocity of sound in the stationary medium, vs be the velocity of the source of sound and vo be the velocity of the observer.
When the source of sound and observer are moving in the same direction, frequency heard by the moving observer is given by
N′=N0(v+vov+vs)
Given,
Velocity of observer vo=36 km/hr or 10 m/s
Velocity of source of sound vs=18 km/hr or 5 m/s
Velocity of sound in stationary medium v=343 m/s
Substituting the given data, we get
N′=1392×(343+10343+5)=1412 Hz
⇒N′=1412 Hz is the frequency heard by the moving observer.