c
\(\mathrm{x}=\frac{(2 \mathrm{n}+1) \lambda \mathrm{D}}{2 \mathrm{a}}\)

For red light, \(x = \frac{{(4 + 1)D}}{{2a}} \times 6500\mathop {\rm{A}}\limits^o \)

For other light, \(x = \frac{{(6 + 1)D}}{{2a}} \times \lambda \mathop {\rm{A}}\limits^o \)

\(x\) is same for each.

\(\therefore 5 \times 6500=7 \times \lambda\)

\( \Rightarrow \lambda  = \frac{5}{7} \times 6500 = 4642.8\mathop {\rm{A}}\limits^o \)