The correct answer is B. Backward process.
Stimulated Brillouin scattering (SBS) is a nonlinear optical process in which a light wave interacts with an acoustic wave to produce a new light wave with a different frequency. The new light wave has a frequency that is lower than the original light wave, and it propagates in the opposite direction.
SBS is a backward process because the new light wave propagates in the opposite direction to the original light wave. This is in contrast to forward stimulated Raman scattering, in which the new light wave propagates in the same direction as the original light wave.
SBS is a nonlinear optical process because it involves the interaction of two light waves. The first light wave is the pump wave, and the second light wave is the Stokes wave. The pump wave is the light wave that is incident on the material, and the Stokes wave is the light wave that is generated by the interaction of the pump wave with the acoustic wave.
SBS is a process that can be used to generate light waves with a different frequency than the original light wave. This can be useful for applications such as optical communications and optical sensing.
The other options are incorrect because they do not describe the process of SBS accurately. Option A, forward process, is incorrect because SBS is a backward process. Option C, upward process, is incorrect because SBS is a downward process. Option D, downward process, is incorrect because SBS is a backward process.