Final answer:
Mitochondria most likely evolved from free-living bacteria through endosymbiosis.
Step-by-step explanation:
The correct answer is a) Free-living bacteria. Mitochondria most likely evolved from free-living bacteria through a process called endosymbiosis. This theory suggests that ancient prokaryotes infected or were engulfed by larger prokaryotic cells, and over time, they developed a symbiotic relationship, becoming organelles inside the larger cells. Mitochondria have their own DNA and resemble a specific group of bacteria in shape, further supporting their origin from free-living bacteria.