The RBC deficiency is deficiency disease of :

The question asks which vitamin deficiency leads to a deficiency disease of red blood cells (RBC). The provided correct option is “Vitamin B₁₂”.

Vitamin B₁₂, also called cobalamin, is a crucial co‑factor in DNA synthesis and the metabolism of homocysteine to methionine. These biochemical pathways are essential for the proper division and maturation of erythrocyte precursors in the bone marrow. When Vitamin B₁₂ is lacking, DNA synthesis is impaired, causing the formation of abnormally large, immature red blood cells (megaloblastic anemia), which is a classic RBC deficiency disease.

Comparing the other options:

• Vitamin B₁ (thiamine) mainly supports carbohydrate metabolism and neuronal function.
• Vitamin B₂ (riboflavin) functions in redox reactions as part of FAD/FMN but does not directly cause RBC pathology.
• Vitamin B₆ (pyridoxine) is involved in amino‑acid metabolism and neurotransmitter synthesis.
• Only Vitamin B₁₂ is directly linked to red blood cell formation and its deficiency results in anemia.

Hence, the deficiency disease of red blood cells is caused by a lack of Vitamin B₁₂.