Correct answer is : 6
\(\sin \theta \simeq \theta \simeq \frac{2 \lambda}{\mathrm{b}}\)
\(=\frac{2 \times 600 \times 10^{-9}}{4 \times 10^{-4}}=3 \times 10^{-3}\ \mathrm{rad}\)
Total divergence \(=(3+3) \times 10^{-3}=6 \times 10^{-3} \ \mathrm{rad}\)