b
Given \(f_{A}=1800 \mathrm{Hz}\)

\({v_{t}=v}\)

\({f_{B}=2150 \mathrm{Hz}}\)

Reflected wave frequency received by \(A.\)

\(f_{A}^{\prime}=?\)

Applying doppler's effect of sound,

\(f^{\prime}=\frac{v_{s} f}{v_{s}-v_{t}}\)

Here, \(v_{t}=v_{s}\left(1-\frac{f_{A}}{f_{B}}\right)=343\left(1-\frac{1800}{2150}\right)\)

\(v_{t}=55.8372 \mathrm{m} / \mathrm{s}\)

Now, for the reflected wave,

\(\therefore \mathrm{f}_{\mathrm{A}}^{\prime}=\left(\frac{\mathrm{v}_{\mathrm{s}}+\mathrm{v}_{\mathrm{t}}}{\mathrm{v}_{\mathrm{s}}-\mathrm{v}_{\mathrm{t}}}\right) \mathrm{f}_{\mathrm{A}}\)

\(=\left(\begin{array}{l}{343+55.83} \\ {343-55.83}\end{array}\right) \times 1800\)

\(=2499.44 \approx 2500 \mathrm{Hz}\)