c
Velocity of wave \(\mathrm{v}=\mathrm{n} \lambda\)

where \(\mathrm{n}=\) frequency of wave \(\Rightarrow \mathrm{n}=\frac{\mathrm{v}}{\lambda}\)

\(\mathrm{n}_{2}=\frac{\mathrm{v}_{2}}{\lambda_{2}}=\frac{396}{100 \times 10^{-2}}=396 \mathrm{Hz}\)

no. of beats \(=n_{1}-n_{2}=4\)