Final answer:
Eukaryotic cells evolved from bacterial prokaryotic cells engulfed by archaean prokaryotic cells, leading to organelles like mitochondria and chloroplasts, supporting the endosymbiotic theory.
Step-by-step explanation:
The development of eukaryotic cells is elegantly explained by the endosymbiotic theory. According to this theory, eukaryotic cells are believed to have evolved from early bacterial prokaryotic cells that were engulfed by archaean prokaryotic cells. This symbiotic relationship eventually led to the prokaryotic cells becoming integrated into the larger archaeal host as organelles, such as mitochondria and chloroplasts, a process that was critical for the evolution of complex eukaryotic cells. Over time, this mutualistic relationship became so integral that the organisms could no longer survive independently, which is a cornerstone in the endosymbiotic theory.
Within this evolutionary framework, the mitochondria would have originated from an ancestral bacterium that was engulfed by a proto-eukaryote. The chloroplasts, responsible for photosynthesis in plants and certain protists, are also thought to have originated from endosymbiotic events involving photosynthetic bacteria. This collaboration between different cellular entities set the stage for the incredible diversity we see in eukaryotic life today. Hence, the correct answer to the student's question is (B) Eukaryotes engulfed prokaryotes.