Final answer:
The strongest evidence for the endosymbiont hypothesis is the similarity of 70S ribosomes in both mitochondria and bacteria, supporting the theory that mitochondria evolved from free-living bacteria entering into a symbiotic relationship with a eukaryotic host.
Step-by-step explanation:
The strongest evidence supporting the endosymbiont hypothesis for the origin of mitochondria and chloroplasts is mitochondrial ribosomes being similar to those of bacteria, indicative of a shared ancestry. The correct answer to the question would be option 'a,' which discusses the similarity of 70S ribosomes found in both mitochondria and prokaryotic cells, supporting the view that mitochondria were once free-living bacteria that entered into a symbiotic relationship with a eukaryotic host cell.
Mitochondrial DNA is also similar to bacterial DNA both in sequence and chromosome structure, but it is not linear as suggested in option 'b.' While Vibrio cholerae and cholera toxin-related studies in option 'c' are important in understanding bacterial pathogenicity, they do not pertain directly to the endosymbiont hypothesis. Finally, option 'd' is incorrect as mitochondria do not have cell walls made of peptidoglycan like bacteria do.