The correct response that directly explains how modern cells demonstrate endosymbiosis is:
An amoeba-like organism engulfed prokaryotes, and they eventually became cells, one inside the other.
This response captures the essence of the endosymbiotic theory, which posits that eukaryotic cells evolved through a process in which prokaryotic cells were engulfed and became organelles, such as mitochondria and chloroplasts, within the host cell.