For what distance is ray optics a good approximation when the aperture is 2 mm wide and the wavelength is 400 nm?

10 m

The correct answer is Option (4) → 10 m

Aperture width a = 2 mm

Fresnel distance $z_f =\frac{a^2}{\lambda}$

$=\frac{\left(2 \times 10^{-3}\right)^2}{400 \times 10^{-9}}=\frac{4 \times 10^{-6}}{400 \times 10^{-9}}$

= 10 m