d
(d) Comparing given equation with standard equation of progressive wave.
The velocity of wave
$v = \frac{{\omega \,({\rm{\,\,Co \,- \,efficient \,\,of \,\,}}t{\rm{)}}}}{{k\,({\rm{\,\,Co\, -\, efficient \,\,of\,\, }}x)}} = \frac{{200\pi }}{{0.5\pi }} = 400\,cm/s$