The best explanation of the endosymbiotic theory among the options provided is:
Amoeba-like cells “ate” single-celled organisms like mitochondria. Instead of digesting them, the mitochondria started to live inside the amoeba-like cells.
This response accurately reflects the core idea of the endosymbiotic theory, which suggests that certain organelles, like mitochondria (and chloroplasts in plants), originated from free-living bacteria that were engulfed by a primitive eukaryotic cell and formed a symbiotic relationship with it.