The RBC deficiency is deficiency disease of :

The correct answer is Vitamin B₁₂.

Explanation:
Red Blood Cell (RBC) deficiency, commonly manifesting as a type of anemia called Pernicious Anemia or Megaloblastic Anemia, is a classic deficiency disease of Vitamin B₁₂ (cobalamin).
Vitamin B₁₂ plays a critical role as a cofactor in DNA synthesis. Specifically, it is required for the synthesis of thymidine, one of the building blocks of DNA. When Vitamin B₁₂ is deficient, cells cannot replicate their DNA properly during cell division.

This impairment in DNA synthesis severely affects rapidly dividing cells, particularly the hematopoietic stem cells in the bone marrow that produce red blood cells. Since DNA replication is stalled but cytoplasmic growth continues, the developing red blood cells become abnormally large and fragile (megaloblasts) and fail to mature into functional red blood cells. This leads to a decreased count of healthy RBCs, causing anemia.

Let's briefly review the roles of the other options:
1. Vitamin B₆ (Pyridoxine): While Vitamin B₆ deficiency can lead to microcytic anemia (due to its role in heme synthesis), RBC deficiency or megaloblastic anemia is predominantly and classically associated with Vitamin B₁₂ (and folate) deficiency.
2. Vitamin B₁ (Thiamine): Deficiency of thiamine leads to Beriberi (affecting the cardiovascular and nervous systems) or Wernicke-Korsakoff syndrome.
3. Vitamin B₂ (Riboflavin): Deficiency of riboflavin causes Ariboflavinosis, which presents with symptoms like lip sores (cheilosis), angular stomatitis, and glossitis.