Answer
b. endosymbiosis.
Work Step by Step
The endosymbiotic theory explains that eukaryotic cells gained mitochondria and chloroplasts through the process of endosymbiosis. Endosymbiosis hypothesizes that eukaryotic cells engulfed a free-living bacterium that developed a symbiotic relationship with its host cell. Therefore, the correct answer would be b. endosymbiosis.
