D

Ray optics is a branch of optics that describes the behavior of light in terms of rays. It is based on the principles that light travels in straight lines and can be reflected and refracted. This theory becomes valid under certain conditions related to the dimensions involved.

Step 1: Consider the relationship between the characteristic dimensions (size of the aperture, obstacle, etc.) and the wavelength of light.
The wavelength of visible light ranges approximately from 400 nm to 700 nm (or 0.4 to 0.7 micrometers).

Step 2: When the characteristic dimensions of objects are much larger than the wavelength of light, the ray optics approximation holds well. This is because, in this scenario, the wave nature of light can be ignored, and it behaves like a ray.

Step 3: If the dimensions are on the same order as the wavelength, diffraction and interference effects become significant, which makes ray optics invalid.

Therefore, the condition under which ray optics is valid is when:
- Characteristic dimensions are much larger than the wavelength of light.

Answer: Therefore, the correct answer is D.