Answer:
Modern day mitochondria evolved from simple prokaryotic cells taken up by larger prokaryotes.
Explanation:
The correct answer as to which best describes the main idea of the endosymbiotic theory would be that modern day mitochondria evolved from simple prokaryotic cells taken up by larger prokaryotes.
The endosymbiotic theory opined that eukaryotic organelles such as mitochondrion and chloroplast evolved from prokaryotic bacteria through endosymbiosis.
Endosymbiosis in itself is a form of mutually beneficial relationship between 2 organisms in which one lives inside the other.
Mitochondrion and chloroplast behave like separate organisms with their own genome, ribosome, and the ability to reproduce. Hence, they are thought to have evolved from prokaryotic cells taken up by larger prokaryotes.
More on endosymbiosis can be found here: https://brainly.com/question/3080991
