The correct response is:
Phagocytosis, a type of endocytosis, was used by a prokaryote to engulf a second prokaryote.
This relates to the theory of endosymbiosis, which suggests that eukaryotes evolved through a process where ancestral prokaryotic cells engulfed other prokaryotic cells. These engulfed cells, which eventually became organelles like mitochondria and chloroplasts, contributed to the complexity of eukaryotic cells.