Endosymbiosis occurs when one organism lives inside the other in a mutually beneficial relationship. The best example from the options given is:
- a – a prokaryotic cell was ingested by a eukaryote and eventually became an organelle.
This option fits the definition because it describes how a prokaryotic organism, like an ancestral bacteria, was engulfed by a eukaryotic cell and became a permanent part of it, eventually evolving into an organelle like mitochondria or chloroplasts. This symbiotic relationship has been widely supported by evidence in cellular biology, particularly through the similarities between mitochondria and certain bacteria.
The other options do not reflect endosymbiosis. In option b, a white blood cell engulfing a microbe is a process called phagocytosis, which is more about defense rather than a symbiotic relationship. Therefore, the best answer highlighting endosymbiosis is option a.