Which process most likely enabled the formation of the early protists? photosynthesis endosymbiosis conjugation binary fission

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.