We use the Rayleigh criterion, which states that the minimum angular separation $\alpha_0$ of two point sources such that ``an observer watching the images on the distant screen can just 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. %(See Bekefi and Barratt pages 578 and 579).
Plugging in the given value for $\lambda$ and $\alpha_0$ , we can find that
$\begin{align*}D = \frac{1.22 \lambda}{\alpha_0} = \frac{1.22 \cdot 5500 \cdot 10^{-10} \mbox{ m}}{8 \cdot 10^{-6}} = 0.0839 \...$
Therefore, answer (C) is correct.

Solution to 1992 Problem 68