Answer
We should use a lens with a small f-number.
Work Step by Step
We can write an expression for the width of the spot:
$w = \frac{2.44~\lambda~f}{D}$
Note that $\frac{f}{D}$ is the f-number.
Therefore, to minimize the width of the spot, we should minimize the f-number.
We should use a lens with a small f-number.
