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.