It is known that Doppler's effect depends on velocity not on distance. When the source is approaching the stationary observer, the apparent frequency heard by the observer is

$n^{\prime }=\frac{v\times n}{v-v_s}=\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}$

But $n^{\prime }>n$.

When the source has crossed the observer, apparent frequency heard by the observer is

$n^{\prime \prime }=\frac{v\times n}{v+v_s}=\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{ }\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}$

But $n^{\prime \prime }<n$.option (d) is correct.