Answer : (c) $\;0.175 \;m$

Explanation :

As time taken is half of the time period , earlier compression , is at a distance of $\;\large\frac{\lambda}{2}\; $ from rarefaction

But $\;\lambda=\large\frac{v}{\nu}=\large\frac{350}{1000}=0.35$

distance (d)$\;=\large\frac{0.35}{2}$$=0.175$