b
The bigger dots represent maximas and the smaller dots represent miniimas.
One of the tube is displaced by $5cm.$
Thus total path difference arises, $x=2 \times 5 c m=10 c m$
Due to this path difference $10$ maximas occur between the end points (minimas).
$\Longrightarrow \frac{\lambda}{2}+9 \lambda+\frac{\lambda}{2}=10$
$\Rightarrow \lambda=1 \mathrm{cm}$
