b
(b) Minimum frequency will be heard, when whistle moves away from the listener.
${n_{\min }} = n\,\left( {\frac{v}{{v + {v_s}}}} \right)$ where $v = r\omega = 0.5 \times 10 = 1\,m/s$
==> ${n_{\min }} = 385\,\left( {\frac{{340}}{{340 + 10}}} \right) = 374Hz.$