Answer : The correct answer is -
mitochondria.
Endosymbiotic theory is an evolutionary theory that describes the origin/evolution of eukaryotic structures, chloroplast and mitochondria from prokaryotic organisms. According to this theory, some bacteria (prokaryotic cell) were considered to live inside larger cells (that is in a symbiotic association). Later those bacteria became the part of larger cells and thought to evolve i to mitochondria and chloroplast, which are now the subcellular structures of eukaryotes.
Thus, mitochondria is the right answer for the question.