d
We know that the apperent frequency

\(\mathrm{f}^{\prime}=\left(\frac{\mathrm{v}-\mathrm{v}_{0}}{\mathrm{v}-\mathrm{v}_{\mathrm{s}}}\right) \mathrm{f}\) from Doppler's effect

where \(v_{0}=v_{s}=30 \mathrm{m} / \mathrm{s},\) velocity of observer and source 

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

\(\because\) Frequency of whistle \((\mathrm{f})=540 \mathrm{Hz}\)

\(\therefore \mathrm{f}^{\prime}=\frac{330+30}{330-30} \times 540=648 \mathrm{Hz}\)