To solve this problem, we need to understand the role of vitamins in the production of red blood cells (RBCs).$\newline$Red blood cell deficiency, often referred to as anemia, can be caused by a deficiency in certain vitamins.$\newline$Vitamin $B_{12}$ is crucial for the production of RBCs. It is involved in DNA synthesis, which is necessary for the formation of red blood cells.$\newline$A deficiency in vitamin $B_{12}$ leads to a specific type of anemia known as pernicious anemia.$\newline$Let's analyze the options:$\newline$• Option 1: Vitamin $B_{12}$ - Correct, as its deficiency leads to RBC deficiency.$\newline$• Option 2: Vitamin $B_{6}$ - While important for hemoglobin production, its deficiency is not directly linked to RBC deficiency.$\newline$• Option 3: Vitamin $B_{1}$ - Known as thiamine, its deficiency causes beriberi, not RBC deficiency.$\newline$• Option 4: Vitamin $B_{2}$ - Known as riboflavin, its deficiency affects energy production, not directly RBC deficiency.$\newline$Therefore, the correct option is Option 1: Vitamin $B_{12}.$