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(D) The frequency $f$ heard by the observer is given by the Doppler effect formula: $f = f_{0} \left( \frac{v_{s}}{v_{s} - v \cos 	heta} ight)$,whe

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(D) The frequency $f$ heard by the observer is given by the Doppler effect formula: $f = f_{0} \left( \frac{v_{s}}{v_{s} - v \cos 	heta} ight)$,where $v_{s}$ is the speed of sound,$v$ is the speed of the source,and $	heta$ is the angle between the velocity vector of the source and the line joining the source to the observer.Before the source reaches the point of closest approach $(t  0$,and the observed frequency $f > f_{0}$. As the source approaches the point of closest approach,$	heta$ increases towards $90^{\circ}$,so $\cos 	heta$ decreases,and $f$ decreases towards $f_{0}$.At the point of closest approach $(t = t_{0})$,$	heta = 90^{\circ}$,$\cos 	heta = 0$,and $f = f_{0}$.After the source passes the point of closest approach $(t > t_{0})$,the source is moving away from the observer,so $	heta$ is obtuse,$\cos 	heta Thus,the frequency $f$ starts above $f_{0}$,decreases to $f_{0}$ at $t = t_{0}$,and continues to decrease below $f_{0}$ for $t > t_{0}$. This behavior is best represented by Graph $D$.

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