Which process describes primary endosymbiosis?

• A. Engulfing of prokaryotic cells by a eukaryotic ancestor, leading to organelles like mitochondria.
• B. Development of organelles from existing eukaryotic cell membranes.
• C. Fusion of two prokaryotic cells to form a eukaryotic cell.
• D. Engulfing of eukaryotic cells by other eukaryotes to form new organisms.

Answer: (A)