In the Endosymbiotic Theory for the origin of eukaryotes, the final step was the engulfing a functional cyanobacterium to produce photosynthetic eukaryotes ("plants"). Why is this stage considered the final step in the sequence of eukaryote evolution?

The Endosymbiotic Theory of eukaryote evolution is known as Symbiogenesis that states the evolution of eukaryotic cells from prokaryotic organisms.

The engulfing a functional cyanobacterium to produce photosynthetic eukaryotes ("plants") is considered as final step of the eukaryote evolution because a functional cyanobacterium closely resemble the chloroplast and related to mitochondria and it maintains a large genome that encodes thousands of proteins.

A photosynthetic eukaryotes ("plants") require cholorplast (green pigment) for photosynthesis and protein synthesis, which all are provided by functional cyanobacterium, after that a eukaryote can functions as an individual eukaryote and not as prokaryote, and hence engulfing functional cyanobacterium is considered as last step of eukaryote evolution.

the correct answer is b