Number of WBCs per mm${}^3$ of human blood is _______.

The correct option is A 8000
White blood cells (WBCs), also called leukocytes, are an important part of the immune system. These cells help to fight against infections by attacking bacteria, viruses, and germs that invade the body. White blood cells originate in the bone marrow but circulate throughout the bloodstream. The number of WBCs per mm${}^3$ of human blood is approximately 8000.

Hence, option A is correct.