B. the similarity in size between the cytosolic ribosomes of prokaryotes and the ribosomes within mitochondria and chloroplasts
Explanation:
Endosymbiosis explains the origin of mitochondria and chloroplast from the prokaryotes that were capable of carrying out aerobic respiration and oxygenic photosynthesis.
These prokaryotes entered the larger cells and eventually lost the ability to survive as individual organisms.
Prokaryotes have 70S ribosomes while eukaryotes have 80S ribosomes. Being semi-autonomous organelles, both mitochondria and chloroplast also have DNA and ribosomes. The ribosomes present in these organelles are of 70S type. This theory is also supported by the presence of circular DNA in bacterial cells and in the mitochondria and chloroplast.
