The correct answer is option A. The hormone erythropoeitin increases the production of red blood cells when oxygen levels are low.
Homeostasis means maintaining a steady state of a particular variable within the body of a living being. That variable may be anything like body temperature, gases like oxygen or carbon dioxide or a body fluid. The body of an organism has various mechanisms to maintain homeostasis. If a particular variable like oxygen in the body of an organism is less than the normal amount then the body produces erythropoietin (EPO) hormone in the kidney which in turn increases the production of red blood cells. The red blood cells have hemoglobin which is responsible for transporting oxygen to the various tissues of the body. So, increased level of RBC implies increased oxygen supply. So, with the help of regulating mechanism the organism produced EPO which increased the level of RBCs in the blood resulting in excess oxygen supply. This is how an organism counters the lack of oxygen with the help of homeostasis.
