Final answer:
The most radiation-resistant organism known is Deinococcus radiodurans, a prokaryote capable of surviving intense ionizing radiation and extreme environmental conditions due to its DNA repair mechanisms.
Step-by-step explanation:
The most radiation-resistant organism known is Deinococcus radiodurans. This prokaryote can tolerate extremely high doses of ionizing radiation, much higher than any human could withstand. Deinococcus radiodurans is even nicknamed 'Conan the Bacterium' because of its hardiness and its ability to repair its DNA even after it's been severely damaged by radiation or heat. This organism is a polyextremophile, able to survive not just high levels of radiation but also extreme desiccation, temperatures, vacuums, and acidity due to unique mechanisms of DNA repair that allow it to reconstruct its chromosome.