The RBC deficiency is deficiency disease of :

The correct option is Vitamin B₁₂.

Explanation:
Red blood cells (RBCs) require Vitamin B₁₂ (also known as cobalamin) for their proper formation, maturation, and division. Vitamin B₁₂ plays a vital role in DNA synthesis during the production of red blood cells in the bone marrow.
A deficiency of Vitamin B₁₂ impairs DNA synthesis, causing the developing red blood cells to grow abnormally large without dividing. This leads to a decrease in the overall number of healthy, functioning red blood cells, a condition known as megaloblastic anemia (or pernicious anemia when due to lack of intrinsic factor). Therefore, a deficiency in RBCs is a direct consequence of Vitamin B₁₂ deficiency.