The correct answer is option 2: A virus.
Polio, also known as poliomyelitis, is caused by the poliovirus. A virus is a microscopic infectious agent that can only replicate inside the cells of a living host. Poliovirus specifically infects the nervous system, causing paralysis and potentially even death.
Option 1: A fungi is incorrect because fungi are a separate kingdom of organisms that are not responsible for causing polio.
Option 3: A worm is also incorrect because worms, such as roundworms or tapeworms, are not responsible for polio either.
Option 4: A bacteria is incorrect because bacteria are single-celled microorganisms that are different from viruses and do not cause polio.
Therefore, the correct answer is option 2: A virus.