Assertion (A) is true because ozone (O3) is a powerful oxidizing agent used for disinfection of water, effectively killing microorganisms like bacteria and viruses. Reason (R) is true because ozone, an allotrope of oxygen, is not radioactive; radioactivity relates to the instability of atomic nuclei. However, ozone's ability to kill microorganisms stems from its strong oxidizing property, not from its non-radioactivity.