(C) It is used as a refrigerant and in fire-extinguishers.
(CFCs are used as refrigerants and in fire-extinguishers, not ozone itself.)
The passage states CFCs — not ozone — are used as refrigerants and in fire extinguishers. All other options correctly describe ozone: it is a deadly poison (A), shields Earth from UV radiation (B), and is formed when free oxygen atoms combine with O₂ (D). Examiners expect you to identify the factual error about ozone vs. CFCs.