Which hormone is primarily responsible for increasing red blood cell count?

Erythropoietin is the hormone primarily responsible for stimulating the production of red blood cells in the bone marrow. It is produced mainly by the kidneys in response to low oxygen levels in the blood, a condition known as hypoxia. When oxygen levels drop, the kidneys release erythropoietin, prompting the bone marrow to increase the production of red blood cells, which are vital for transporting oxygen throughout the body.

This mechanism is crucial for maintaining adequate oxygen delivery, especially in situations like high altitude or after significant blood loss. While other hormones mentioned play various important roles in metabolism and bodily functions, erythropoietin specifically targets the production of red blood cells, making it the correct choice in this context.