Answer
0.22 mm
Work Step by Step
Given: $\lambda=589\,nm=589\times10^{-9}\,m$
$D=150\,cm=150\times10^{-2}\,m$
$\beta=4.0\,mm=4.0\times10^{-3}\,m$
Recall: $\beta=\frac{\lambda D}{d}$ or $d=\frac{\lambda D}{\beta}$
Result: $d=\frac{589\times10^{-9}\,m\times150\times10^{-2}\,m}{4.0\times10^{-3}\,m}=0.00022\,m=0.22\,mm$
