Missing question:
A. add 0.833 mL 12 M HCl stock solution to 99.167 mL water.
B. add 0.833 mL 12 M HCl stock solution to 49.167 mL water.
C. add 0.833 mL 12 M HCl stock solution to 199.167 mL water.
c₁ - original concentration of the solution, before it gets diluted.
c₂ - final concentration of the solution, after
dilution.
V₁ - volume to be diluted.
V₂ - final volume after dilution.
c₁ · V₁ = c₂ · V₂.
c₁(HCl) = 12.0 M.
c₂(HCl) = 0.100 M.
V₂(HCl) = 3 · 15.0 mL = 45.0 mL; for three experiments.
V₁(HCl) = c₂ · V₂ / c₁.
V₁(HCl) = 0.375 mL.
Make proportion: 0.375 mL : 0.833 mL = 45 mL : V.
V = 100 mL.