Final answer:
Erythropoietin is produced by the kidneys in response to low blood oxygen levels, and it stimulates the bone marrow to produce erythrocytes.
Step-by-step explanation:
The stimulus for the production of erythropoietin (EPO) is low oxygen levels in the blood. Specifically, EPO is a glycoprotein hormone secreted by the interstitial fibroblast cells of the kidneys when they detect low blood oxygen levels. The role of EPO is to stimulate the bone marrow to produce more erythrocytes, commonly known as red blood cells (RBCs), which carry oxygen throughout the body. Therefore, the correct answer to the question is B. Low blood oxygen in the kidneys.