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