Correct choice is (c) It regulates the work of free radicals
The best I can explain: When radiation strikes the cells, it ionizes the water molecules and they break into H and OH ions. These ions are charged with energy due to the radiations and they attack the DNA of the cancerous cells and damages them. In the presence of oxygen, this process is accelerated and also the damage inflicted on the DNA is brought under control. The oxygen reacts with the free hydrocarbon particles and fills up the gaps in DNA allowing it to recover properly.