A listener moves toward stationary source with constant speed and crosses him, then correct graph between apparent frequency (f') with time (t) is

velocities are taken along the line joining the source and listener when listener moves towards the source then apparent frequency is more than the actual frequency$n\text{'}=\left(\frac{v+v_0}{v}\right)n$ and after crossing it moves away from the source apparent frequency decreases $n\text{'}=\left(\frac{v-v_0}{v}\right)n$