How does the endosymbiotic theory explain the presence of mitochondrial DNA?

• A. Mitochondria evolved from the nuclear DNA of eukaryotic cells.
• B. Mitochondria acquired DNA from viral infections.
• C. Mitochondria developed DNA independently after the formation of eukaryotic cells.
• D. Mitochondria were once autonomous cells that merged with eukaryotic cells, retaining their own DNA.

Answer: (D)