What Ratio of [NH₄⁺]/[NH₃] Would Provide a Buffer of PH

What ratio of [NH₄⁺]/[NH₃] would provide a buffer of pH low enough to avoid precipitation of Co(OH) ₂ [Ksp = 2.0 × 10^-14] from a 0.50 M Co²⁺ solution? [Kb for NH₃ = 1.8 × 10^-5]

A) 1/50
B) 1/90
C) 1/1
D) 90/1
E) 50/1