A train whistling at constant frequency is moving towards a station at a constant speed $v$.The train goes past a stationary observer on the station. The frequency $n\text{'}$of the sound as heard by the observer is plotted as a function of time t (figure.). Identify the expected curve.

Whistling train is the source of sound $v_s=V$. Before crossing a stationary observer on station, frequency heard is $n\text{'}=\frac{vn}{(v-v_s)}=\frac{vn}{v+V}$  constant and  $n\text{'}>n.$
Here, v is velocity of sound in air and n is actual frequency of whistle.
After crossing the stationary observer, frequency heard is $n\text{'}=\frac{vn}{(v+v_s)}=\frac{vn}{v+V}$  constant and $n\text{'}>n.$ 
Therefore, the expected curve is (c).