a
(a) Comparing the given equation with $y = a\sin \left( {\omega t - kx} \right)$
We get $\omega = 3000\,\pi $==> $n = \frac{\omega }{{2\pi }} = 1500\,Hz$
and $k = \frac{{2\pi }}{\lambda } = 12\pi \Rightarrow \lambda = \frac{1}{6}m$
So, $v = n\lambda \Rightarrow v = 1500 \times \frac{1}{6} = 250\,m/s$