Answer: Endosymbiont theory.
- A Cell theory
- B Endosymbiont theory
- C Germ theory
- D Fluid mosaic model
Correct answer: B. Endosymbiont theory
Explanation: Endosymbiont theory (Lynn Margulis): mitochondria and chloroplasts were once free-living prokaryotes engulfed by ancestral eukaryotic cells.
Animal cell structure with major organelles labeled. Image: LadyofHats (Mariana Ruiz), Public Domain, via Wikimedia Commons.
Concept context
Cell theory, prokaryotic vs eukaryotic cells, organelles and their functions. Foundation of all biology.