Two sources emitting sound in phase gives interference at P, as shown. Then for different wavelengths $\lambda$, choose the correct alternative 

 

Here, $\mathrm{\Delta }x=6-4=2m$ for constructive interference $\mathrm{\Delta }x=n\lambda$

or $\lambda =\frac{\mathrm{\Delta }x}{n}=2\mathrm{m},1\mathrm{m},\frac{2}{3}\mathrm{m},\frac{1}{2}\mathrm{m},\frac{2}{5}\mathrm{m}\text{ etc. }$

and for destructive interference $\mathrm{\Delta }x=(2n+1)\frac{\lambda }{2}$

or $\lambda =\frac{2\mathrm{\Delta }x}{2n+1}=4\mathrm{m},\frac{4}{3}\mathrm{m},\frac{4}{5}\mathrm{m},\frac{4}{7}\mathrm{m}\text{ etc. }$