Option 1 -> The passage mentions Mars exploration but never suggests these microbes evolved from Martian microorganisms.
Option 2 -> The passage explicitly states DNA staining confirmed the microbes' presence and they had intact cell walls, contradicting this option.
Option 3 -> The passage directly states "These microbes, primitive in evolutionary terms, offer clues about Earth's earliest life forms and their evolution," indicating their primitive nature relates to resembling early life.
Option 4 -> The rock is 2-billion-year-old, which is extremely ancient, not relatively young.
Hence, Option 3 -> The microbes are found in 2-billion-year-old rock, making them among the oldest known life forms discovered. Their ancient origin and survival in extreme conditions provide insights into what Earth's earliest organisms might have been like and how they adapted to harsh environments -> correct