The correct answer is: C. High optical power densities.
Raman scattering is a type of inelastic scattering of light by a material. It occurs when a photon interacts with a molecule and transfers some of its energy to the molecule, causing it to vibrate. The scattered photon has a lower energy than the incident photon, and the difference in energy is absorbed by the molecule.
Brillouin scattering is a type of elastic scattering of light by a material. It occurs when a photon interacts with a sound wave in the material. The scattered photon has the same energy as the incident photon, but its direction is changed by the sound wave.
Both Raman and Brillouin scattering are usually observed at high optical power densities. This is because the probability of a photon interacting with a molecule or a sound wave increases with the intensity of the light.
Option A is incorrect because Raman and Brillouin scattering are usually observed at high optical power densities, not low optical power densities.
Option B is incorrect because Raman and Brillouin scattering are usually observed at high optical power densities, not medium optical power densities.
Option D is incorrect because Raman and Brillouin scattering are not usually observed at threshold power densities. Threshold power densities are the minimum power densities required to initiate a particular process, such as lasing. Raman and Brillouin scattering do not require a threshold power density to occur.