We use the Rayleigh criterion, which states that the minimum angular separation $\alpha_0$ of two points sources such that an observer can tell them apart
is for a circular aperature with diameter $D$
$\begin{align*}\alpha_0 = \frac{1.22 \lambda}{D}\end{align*}$
where $\lambda$ is the wavelength of the light.
Plugging in the given value for $\lambda$ and $\alpha_0$ , we can find that
$\begin{align*}D = \frac{1.22 \lambda}{\alpha_0} = \frac{1.22 \cdot 600 \cdot 10^{-9} \mbox{ m}}{3 \cdot 10^{-5}} = 0.0839 \mb...$
Therefore, answer (B) is correct.

Solution to 2001 Problem 13