The correct answer is option 1: Bacterium. Anthrax is a disease caused by the bacterium Bacillus anthracis. This bacterium forms spores, which are highly resistant and can survive for long periods in the environment. Anthrax can affect both humans and cattle, as well as other animals like sheep and goats.

Option 2: Virus is not the correct answer because anthrax is caused by a bacterium, not a virus. Viruses are distinct infectious agents that are smaller and have a different structure compared to bacteria.

Option 3: Protozoan is also not the correct answer. Protozoa are single-celled organisms that are eukaryotic, whereas anthrax is caused by a bacterium, which is a prokaryotic organism.

Option 4: Fungus is not the correct answer either. Fungi are a separate group of organisms that include molds and yeasts. Anthrax is caused by a bacterium, not a fungus.

In summary, anthrax is caused by the bacterium Bacillus anthracis, making option 1 the correct answer.