\(v_{0}=800 \mathrm{Hz}\)

Speed of source, \(v_{s}=15 \mathrm{m} / \mathrm{s}\)

Speed of sound in air, \(\mathrm{v}=330 \mathrm{m} / \mathrm{s}\)

Apparent frequency of sound at the cliff \(=\) Frequency heard by observer \(=v\)

Using Doppler's effect of sound

\(v=\left(\frac{v}{v-v_{S}}\right) v_{0}=\frac{330}{330-15} \times 800\)

\(=\frac{330}{315} \times 800=838.09 \mathrm{Hz} \approx 838 \mathrm{Hz}\)