When the count of WBC drops below the optimum level of blood it is known as

The correct option is leukopenia.

To understand why this is the correct answer, let us break down the medical terminology and look at the function of white blood cells (WBCs):

1. Understanding the prefix and suffix:
- "Leuko-" refers to leukocytes, which is the scientific name for white blood cells (WBCs). White blood cells play a vital role in the body's immune system, helping to fight off infections and diseases.
- The suffix "-penia" is derived from Greek and means a lack, deficiency, or reduction in number.

2. Combining the terms:
Combining "leuko-" (white blood cells) and "-penia" (deficiency/decrease) gives us leukopenia, which literally translates to a decrease or drop in the count of white blood cells below the normal or optimum level.

3. Why the other options are incorrect:
- Leukemia: This is a type of blood cancer characterized by the rapid and abnormal production of white blood cells, usually leading to an abnormally high (rather than low) count of dysfunctional WBCs.
- Anemia: This refers to a condition where there is a deficiency in red blood cells (RBCs) or hemoglobin, not white blood cells.

Therefore, a drop in the WBC count below the optimum level is specifically known as leukopenia.