b
Car is the source and the hill is observer.

Frequeney heard at the hill, \(v_{1}\)

\(\therefore v_{1}=\frac{v \times v}{(v-V)}=\frac{600 \times 330}{330-30}\)

Now for reflection, the hill is the source and the driver the observer.

\(\therefore v_{2}=v_{1} \times \frac{(330+30)}{330}\)

\(\Rightarrow v_{2}=\frac{600 \times 330}{300} \times \frac{360}{330} \Rightarrow v_{2}=720 \mathrm{Hz}\)