c
\(Case\, 1:\) When source is moving towards stationary listener

apparent frequency \(\eta^{\prime}=\eta\left(\frac{\mathrm{v}}{\mathrm{v}-\mathrm{v}_{\mathrm{s}}}\right)\)

\(=500\left(\frac{340}{336}\right)=506 \mathrm{Hz}\)

\(Case\, 2:\) When source is moving away from the stationary listener

\(\eta^{\prime \prime}=\eta\left(\frac{v}{v+v_{s}}\right)=500\left(\frac{340}{344}\right)=494 \mathrm{Hz}\)

In case \(1\) rumber of beats heard is \(6\) and in

\(case\, 2\) number of beats heard is \(18\) therefore frequency of the source at \(\mathrm{B}=512 \mathrm{Hz}\)