What Is the Predominant Atomic or Molecular Mechanism That Produces

What is the predominant atomic or molecular mechanism that produces the light we see from emission nebulae?

A) Radio energy from embedded stars excites atoms to high atomic states, and spectral lines are produced as these atoms return to their unexcited states.
B) Radiation from nearby stars heats the gas, and the gas then emits a continuum spectrum appropriate to its temperature.
C) Photons of UV and X radiation from very hot embedded stars accelerate electrons by collision, and these accelerating electrons radiate at all wavelengths.
D) UV light from hot stars ionizes atoms, and the subsequent recombination of electrons with these ions produces spectral lines.