Which of these observations gives the most support to the endosymbiotic theory for the origin of eukaryotic cells?
a. the existence of structural and molecular differences between the plasma membranes of prokaryotes and the internal membranes of mitochondria and chloroplasts
b. the similarity in size between the cytosolic ribosomes of prokaryotes and the ribosomes within mitochondria and chloroplasts
c. the size disparity between most prokaryotic cells and most eukaryotic cells the observation that some eukaryotic cells lack mitochondria

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.