The correct answer is option 1: Virus.

Rabies is a highly fatal disease that affects mammals, including humans. It is primarily caused by a virus known as the Rabies virus. The virus belongs to the Rhabdoviridae family and is transmitted through contact with the saliva or nervous tissue of an infected animal, usually through a bite.

Option 2: Bacteria is an incorrect choice. Rabies is not caused by bacteria but by a virus. Bacteria are single-celled microorganisms that can cause various diseases, but they are not responsible for causing rabies.

Option 3: Protozoan is also an incorrect choice. Protozoans are single-celled eukaryotic organisms that can cause diseases such as malaria and amoebic dysentery, but they are not associated with causing rabies.

Option 4: Nematode worm is another incorrect choice. Nematode worms are multicellular organisms that can be parasites, but they do not cause rabies. Rabies is caused by a virus, not a nematode worm.

In conclusion, the correct answer is that rabies is caused by a virus, specifically the Rabies virus.