Correct answer is : 2
\(\mathrm{d}=1 \mathrm{~mm}, \mathrm{D}=1 \mathrm{m}, \lambda=500 \mathrm{~nm}\)
\(10\left(\frac{\lambda \mathrm{D}}{\mathrm{d}}\right)=\frac{2 \lambda \mathrm{D}}{\mathrm{a}}\)
\(\mathbf{a}=\frac{\mathrm{d}}{5}\)
\(=\frac{10 \times 10^{-4} \mathrm{~m}}{5}\)
\(=2 \times 10^{-4}\)