d
(d)
Velocity of sound $=330 \,m / s$
Frequency of incident sound on wall $\left(f_1\right)=\frac{v}{v-v_s} \times f$
Frequency of sound observed by driver $\left(f_2\right)=\frac{v+v_s}{v} \times f_1$
or $f_2=\frac{v+v_s}{v-v_s} \times 600$
or $f_2=\frac{360}{300} \times 600$
or $f_2=720 \,Hz$