The causative agent of Poliomyelitis is

Poliomyelitis, also known as polio, is a viral disease that affects the nervous system. The question is asking about the causative agent, which means the organism that is responsible for causing the disease. The options given are virus, fungus, protozoan, and bacterium.

The correct answer is option A: Virus. Poliomyelitis is caused by the poliovirus, which is a type of RNA virus. It is highly contagious and spreads through contact with infected fecal matter or respiratory secretions. The virus attacks the nervous system, leading to paralysis, muscle weakness, and even death in severe cases.

Fungi, protozoa, and bacteria are all different types of organisms, but they are not responsible for causing poliomyelitis. Fungi are usually associated with diseases like ringworm and athlete's foot, while protozoa can cause illnesses like malaria and amoebic dysentery. Bacteria can cause diseases like tuberculosis and cholera.

In summary, the correct answer to the question is option A: Virus, specifically the poliovirus. To learn more about poliomyelitis and other infectious diseases, please read the relevant sections of the recommended textbooks.