Calculate the Minimum Concentration of Mg²⁺ That Must Be Added

Calculate the minimum concentration of Mg²⁺ that must be added to 0.10 M NaF in order to initiate a precipitate of magnesium fluoride. [K_sp(MgF₂) = 6.9 × 10^-9]

A) 1.4 × 10^{7 M}
B) 6.9 × 10^{-9 M}
C) 6.9 × 10^{-8 M}
D) 1.7 × 10^{-7 M}
E) 6.9 × 10^{-7 M}