Abnormal rise in number of RBC count is known as

The correct option is polycythemia.

Here is a step-by-step breakdown of why this option is correct and what the other terms mean:

1. Understanding Polycythemia:
Polycythemia is a medical condition characterized by an abnormal increase in the number of red blood cells (erythrocytes) in the circulatory system. This elevation can cause the blood to become thicker (more viscous), which can slow down blood flow and increase the risk of blood clots. The term originates from the Greek words poly (many), cyto (cell), and hemia (blood).

2. Analyzing the Other Options:
Anaemia: This is the opposite of polycythemia. Anaemia is a condition characterized by a deficiency or lower-than-normal count of red blood cells or hemoglobin in the blood, leading to reduced oxygen transport to the body's tissues.
Leukemia: This is a type of cancer that affects the blood-forming tissues in the body, including the bone marrow and the lymphatic system. It is characterized by an abnormal, uncontrolled rise in the number of white blood cells (leukocytes), not red blood cells.

Therefore, an abnormal rise in the number of RBC count is specifically known as polycythemia.