Final answer:
The mitochondrion is the organelle believed to have evolved from aerobic prokaryotes engulfed by ancestral eukaryotic cells, which helped the cell use oxygen to release energy.
Step-by-step explanation:
The organelle whose ancestor was likely engulfed by primitive eukaryotes to help the cell survive in an oxygen-rich atmosphere is the B) Mitochondrion. According to the Endosymbiotic Theory, initially proposed by Lynn Margulis, mitochondria evolved from free-living, aerobic prokaryotes that were engulfed by ancestral eukaryotic cells. This relationship allowed the host cell to effectively use oxygen to release energy from nutrients. Mitochondria have many similarities to bacteria, such as being surrounded by two membranes and having their own DNA, which supports the theory that mitochondria and chloroplasts originated from endosymbiotic events.