What is the optical output predominantly produced by recombination?

The optical output predominantly produced by recombination is spontaneous emission. During recombination, electrons in a semiconductor material fall from a conduction band to fill holes in the valence band. This process releases energy that can take the form of light (photons).

Spontaneous emission is a random process whereby the emitted photons are generated without any external stimulation, and it's a fundamental mechanism in light-emitting devices such as LEDs and semiconductor lasers when they are in an active region. The emitted light is due to the energy difference between the conduction and valence bands being released as electromagnetic radiation.

The other options represent different phenomena that are not directly related to the recombination process that generates light. Reflection refers to light bouncing off a surface, absorption involves the uptake of light energy by a material, and transmission describes light passing through a medium. None of these processes generate light through recombination, which is specifically tied to spontaneous emission.