To solve this problem, we need to identify the source of the erythropoietin hormone, which stimulates the formation of red blood cells (R.B.Cs). Let's analyze each option:$\newline \bullet$Option 1: Alpha cells of pancreas$\newline$ - Alpha cells of the pancreas primarily produce glucagon, a hormone involved in glucose metabolism.$\newline$ - They do not produce erythropoietin.$\newline \bullet$Option 2: The cells of rostral adenohypophysis$\newline$ - The rostral adenohypophysis, or anterior pituitary gland, produces several hormones such as growth hormone, prolactin, and ACTH.$\newline$ - It does not produce erythropoietin.$\newline \bullet$Option 3: The cells of bone marrow$\newline$ - Bone marrow is the site of blood cell production, including red blood cells, but it does not produce erythropoietin.$\newline$ - Erythropoietin acts on the bone marrow to stimulate R.B.C. production.$\newline \bullet$Option 4: Juxtaglomerular cells of the kidney$\newline$ - Juxtaglomerular cells are specialized cells in the kidney that produce erythropoietin in response to low oxygen levels in the blood.$\newline$ - This hormone then stimulates the bone marrow to increase the production of red blood cells.$\newline$Therefore, the correct answer is Option 4: Juxtaglomerular cells of the kidney.