c
Here, $v_{0}=400 H z, v=340 m s^{-1}, v_{w}=10 m s^{-1}$
As there is no relative motion between the source and observer, therefore the frequency of sound remains unchanged.
Hence, the frequency of sound as heard by an observer standing on the platform is $v_{0}=400 H z$
As the wind is blowing in the direction of sound, therefore, for an observer standing on platform,
Speed of sound $=v+v_{w}=340+10=350 \mathrm{ms}^{-1}$