Erythropoietin hormone which stimulates R.B.C. formation is produced by:

The correct option is Juxtaglomerular cells of the kidney.

Erythropoietin (EPO) is an essential peptide hormone responsible for regulating erythropoiesis, which is the process of red blood cell (R.B.C.) production in the bone marrow.

In response to low cellular oxygen levels (hypoxia) in the blood, specialized cells known as juxtaglomerular cells (or interstitial cells of the peritubular capillary network) in the kidneys synthesize and secrete erythropoietin.

Once released into the bloodstream, erythropoietin travels to the red bone marrow, where it stimulates the maturation and proliferation of erythroid progenitor cells into mature R.B.C.s, thereby increasing the oxygen-carrying capacity of the blood.

Let us analyze why the other options are incorrect:
- The cells of bone marrow: While bone marrow is the target organ where erythropoiesis takes place, erythropoietin itself is produced by the kidneys.
- Alpha cells of pancreas: Alpha cells produce glucagon, a hormone involved in elevating blood glucose levels.
- The cells of rostral adenohypophysis: The adenohypophysis (anterior pituitary) synthesizes hormones such as growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), prolactin, and gonadotropins, but not erythropoietin.