Bichromatic light of wavelengths ${\lambda }_1$ = 5000 Å and ${\lambda }_2$ = 7000 Å are used in YDSE. Then,

 

For overlapping of maxima

  $$\begin{gathered} \frac{n_1{\lambda }_{1}D}{d}=\frac{n_2{\lambda }_{2}D}{d} \\ or \frac{n_1}{n_2}=\frac{{\lambda }_2}{{\lambda }_1}=\frac{7}{5},\frac{14}{10},\dots \end{gathered}$$

$\Rightarrow$14th order maxima of ${\lambda }_1$ will coincide with 10th order maxima of ${\lambda }_2$.

For overlapping of minima

$$\begin{gathered} \frac{\left(2n_1-1\right){\lambda }_{1}D}{2d}=\frac{\left(2n_2-1\right){\lambda }_{2}D}{2d} \\ \frac{2n_1-1}{2n_2-1}=\frac{{\lambda }_2}{{\lambda }_1} \\ =\frac{7}{5}=\frac{21}{15} \end{gathered}$$

therefore, option 3 with n₁=11 and n₂=8 gives this ratio.