The correct answer is option 3, virus. Chicken pox is caused by a virus called varicella-zoster virus, which belongs to the herpesvirus family. Viruses are tiny infectious agents that can only replicate inside living cells of hosts, such as humans. Unlike protozoa, which are single-celled organisms, and bacteria, which are single-celled microorganisms, viruses are not considered living organisms as they lack cellular structures and metabolic processes. Fungi, on the other hand, are a distinct group of eukaryotic organisms that can cause various diseases, but they are not responsible for chicken pox. Therefore, it is important to understand that chicken pox is specifically caused by a viral infection and not by protozoa, bacteria, or fungi.