Answer: Endosymbiosis
Explanation:
During the 1980s a theory was proposed to explain the origin of chloroplast and mitochondria from the permanent resident prokaryotes.
according to this theory, large prokaryotes engulfed smaller prokaryotes, instead of digesting the smaller one both entered into a type of relationship known as mutualism where both organism were benefited.
The larger prokaryotes gained more energy which provided protomitochondrion and excessive sugar lead to the formation of chloroplast.