d
\(\text { Given }: y_{1}=4 \sin 600 \pi t, y_{2}=5 \sin 608 \pi t\)

\(\therefore \quad \omega_{1}=600 \pi\) or \(2 \pi v_{1}=600 \pi\) or \(v_{1}=300\)

\(A_{1}=4\)

and \(\omega_{2}=608 \pi\) or \(2 \pi \mathrm{v}_{2}=608 \pi\) or \(\mathrm{v}_{2}=304\)

\(A_{2}=5\)

Number of beats heard per second

\(=v_{2}-v_{1}=304-300=4 \)

\(\frac{I_{\max }}{I_{\min }}=\frac{\left(A_{1}+A_{2}\right)^{2}}{\left(A_{1}-A_{2}\right)^{2}}=\frac{(4+5)^{2}}{(4-5)^{2}}=\frac{81}{1}\)