The correct answer would be mitochondria or chloroplast.
Endosymbiosis refers to the evolution theory which explains the origin of eukaryotic cells from prokaryotic cells.
It is believed that cell organelles like chloroplast, mitochondria et cetera were free-living early prokaryotic cells which were taken inside the other cells by the process of endosymbiosis. It led to the origin of eukaryotic cells which were able to perform photosynthesis.
Thus, Keiko should mark chloroplast or mitochondria in the diagram.
