a
Velocity of the wave is given by$;$
$v=\frac{\lambda}{T}$
$T=\frac{1}{f}$
$\boldsymbol{v}=\lambda f$
From the graph, we observe that $\lambda$ is $0.4 c m$
$f$ is given to be $250 \mathrm{Hz}$, converting the wavelength to SI, and multiplying we get $v=1 \mathrm{m} / \mathrm{s}$