A police car with a siren of frequency 8 kHz is moving with uniform...

Apparent frequency of sound heard by car driver (observer) reflected from the building will be $$\begin{aligned}f^{\prime} & =f\left(\frac{v+v_0}{v-v_s}\right)\\ & =8\left(\frac{320+10}{320-10}\right)\\ & =8.5 \mathrm{kHz} \end{aligned}$$\therefore$ Correct option is (a). Analysis of Question (i) Question is simple. (ii) Driver will listen two sounds, direct and reflected. Direct sound will be of 8 \mathrm{kHz} as driver has no relative motion with the car. But reflected sound is of increased frequency because driver and image of car both are approaching towards each other.