b
No. of beats per second $=\frac{9}{3}=3 \mathrm{s}^{-1}$
No. of beats per second $=v_{1}-v_{2}$
$3=\frac{\mathrm{v}}{\lambda_{1}}-\frac{\mathrm{v}}{\lambda_{2}}=\mathrm{v}\left[\frac{1}{\lambda_{1}}-\frac{1}{\lambda_{2}}\right]$
$\frac{3}{300}=\frac{1}{2}-\frac{1}{\lambda_{2}}$
$\frac{1}{\lambda_{2}}=\frac{1}{2}-\frac{1}{100}=\frac{50-1}{100}=\frac{49}{100}$
$\lambda_{2}=\frac{49}{100}=2.04 \mathrm{\,m}$