d
(d)Distance of \({n^{th}}\) dark fringe from central fringe
\({x_n} = \frac{{(2n - 1)\lambda \,D}}{{2d}}\)
\(\therefore \,\,\,{x_2} = \frac{{(2 \times 2 - 1)\lambda \,D}}{{2d}} = \frac{{3\lambda \,D}}{{2d}}\)
\( \Rightarrow 1 \times {10^{ - 3}} = \frac{{3 \times \lambda \times 1}}{{2 \times 0.9 \times {{10}^{ - 3}}}} \Rightarrow \lambda = 6 \times {10^{ - 5}}\,cm\)