A railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer beside the railway
track. Which of the following graphs best represents the variation of frequency of the sound (n) heard by the observer with time (t)?

Apparent frequency is dependent on the velocity of the source and observer and is independent of the distance. Since the speed of the train is constant, frequency will be constant till the train crosses the observer. 

Once it crosses the observer, the frequency changes but again stays constant for the rest of the time.