b
Let, $n_{1}$ and $n_{2}$ be the frequencies of two waves.
$\lambda_{1}$ and $\lambda_{2}$ be the wavelengths of two waves.
$v$ be the speed of sound.
The number of beats per second are
$n_{1}-n_{2}=6$
$\frac{v}{\lambda_{1}}-\frac{v}{\lambda_{2}}=6$
$\frac{v}{1}-\frac{v}{1.02}=6$
$\frac{0.02 v}{1.02}=6$
$0.02 v=(6)(1.02)$
$v=\frac{6.12}{0.02}=306 \mathrm{ms}^{-1} \approx 300 \mathrm{m} / \mathrm{s}$