Complete burning of plant material returns carbon primarily to the
a. herbivores.
b. water.
c. vegetation.
d. atmosphere.

I think the correct answer from the choices listed above is option D. Complete burning of plant material returns carbon primarily to the atmosphere. Burning of plant materials have products of carbon dioxide and water vapor which goes directly to the atmosphere. 

Eukaryotic cells are theorized to have evolved from prokaryotes called the endosymbiotic theory. this explains that the most primitive eukaryotic cell engulfed a prokaryotic cell (by the process of phagocytosis) that is capable of cellular respiration and another prokaryotic cell that is capable of photosynthesis. these prokaryotic cells eventually became organelles and these organelles are the mitochondria and the chloroplast