Answer:
Chloroplast
Explanation: The endosymbiotic theory of organelle evolution is the theory that our organelles (small parts inside of a cell like mitochondria and cholorplasts) evolved when primitve cells ingested smaller cells with certain functions. Imagine a big cell, consuming a small cell that can perform photosynthesis. If it doesn't digest that cell, when the smaller cell produces energy using the sun, the larger cell will also benefit from the energy being produced. Over time, the cells replicate together and that mini cell, becomes known as an organelle.
Similarly, mitochondria in humans are believed to have evolved from smaller prokaryotic cells that were among the few cells able to use oxygen during early earth. The larger cell that consumed them benefit from having this oxygen consuming cell inside of it because oxygen may have been toxic to it.
Circular DNA and 70s ribosomes were the first clues that these organelles were evolved from prokaryotic cells. It is odd that parts of the cell have their own genome, but it was even more odd that their genomes had different characteristics. Eukaryotic cells have linear dna, and prokaryotic cells have circular dna like chloroplasts and mitochondria. Prokaryotes and eukaryotes also have different ribosomal sizes. This is somewhat related to the fact that we inherit our mitochondria from our mothers. Mitochondria are found in the egg and we can trace maternal ancestry using mitochondrial DNA.