The right answer is endosymbiosis.
By the beginning of the 20th century, researchers thought that plastids and mitochondria could come from bacteria. These would have been ingested by primitive cells and live within them in symbiosis.
This endosymbiotic theory of the origin of plastids and mitochondria became perfectly plausible when it was discovered (1950-1960) that these organelles contained DNA and ribosomes.
Answer: Endosymbiosis
It was proposed that the eukaryotic cells have evolved from the symbiotic association of two or more bacterial cells. The bacterial cells which were smaller in size might be engulfed by the larger ones. This way all the beneficial organelles evolved together as a single cell. This is how protist might have evolved. This was termed as "endosymbiotic relationship".
