The siren is moving towards the cliff, so the cliff acts as a stationary observer receiving sound from a moving source. The apparent frequency $f'$ reaching the cliff is given by the Doppler effect formula: $f' = f_0 \left(\frac{v}{v - v_s}\right)$ where: $f_0 = 800\text{ Hz}$ (actual frequency of the siren) $v = 330\text{ ms}^{-1}$ (speed of sound) $v_s = 15\text{ ms}^{-1}$ (speed of the source)

$f' = 800 \times \left(\frac{330}{330 - 15}\right) = 800 \times \frac{330}{315} = 800 \times \frac{22}{21} \approx 838.09\text{ Hz}$

The cliff then acts as a stationary source of sound, reflecting it back to the stationary observer. The frequency of the echo heard by the observer will be the same as the frequency received by the cliff. Therefore, the frequency of the echo heard by the observer is approximately $838\text{ Hz}$.