Polio is caused by a virus. Option 1, Bacterium, is incorrect because bacteria are different from viruses. Bacteria are single-celled organisms that can cause various infections, while viruses are much smaller and need a host to multiply. Option 2, Fungus, is also incorrect because fungi are a separate group of organisms that can cause different types of infections. Option 4, Insect, is not related to the cause of polio at all. Therefore, the correct answer is option 3, Virus. The poliovirus specifically attacks the nervous system, leading to paralysis in some cases. It is primarily transmitted through contaminated water or food and can spread quickly in areas with poor sanitation. Alert - The correct answer should be option 3, Virus.