The theory of endosymbiosis states that the eukaryotic cells evolved from the prokaryotic cells. According to this theory, a large bacterium ingested a small bacterium and then they evolved together working as a single unit. This can be shown through a model by demonstrating a big bacteria ingesting the small one. The small bacteria came to be cell organelles, such as mitochondria and chloroplasts in the eukaryotic cells with their own DNA.
Hence, the correct answer is 'Option A - show the big bacteria ingesting the small one'.