The best response that illustrates how modern cells demonstrate endosymbiosis is:
An amoeba-like organism engulfed prokaryotes, and they eventually became cells, one inside the other.
This option highlights the key aspect of endosymbiosis, which is the engulfing of one cell type by another, leading to a mutually beneficial relationship and the eventual integration of the engulfed prokaryotes into the host cell, contributing to the evolution of eukaryotic cells.