At high altitude, the RBC in human blood
The correct option is D Increase in number.
At high altitudes, there is a decrease in oxygen hemoglobin saturation. This hypoxic condition causes hypoxia-inducible factor 1 (HIF- 1) to become stable and stimulates the production of erythropoietin (EPO), a hormone secreted by the kidneys. EPO stimulates red blood cell production from bone marrow in order to increase hemoglobin saturation and oxygen delivery. Thus, the total number of RBC increases.