b
\(\beta=\frac{\lambda D}{d}\)
\(\Delta \beta=\frac{\lambda}{ d } \Delta D\)
\(\lambda=\frac{\Delta \beta \cdot d}{\Delta D }\)
\(=\frac{3 \times 10^{-5} \times 1 \times 10^{-3}}{5 \times 10^{-2}}\)
\(=60 \times 10^{-8}=600 \times 10^{-9} \,m\)
\(=600 \,nm\)