The best response illustrating how modern cells demonstrate endosymbiosis is:
An amoeba-like organism engulfed prokaryotes, and they eventually became cells, one inside the other.
This describes the theory of endosymbiosis, where certain organelles, such as mitochondria and chloroplasts, are thought to have originated from free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells, leading to a mutually beneficial relationship.