Answer is: cobalt(II) hydroxide.
Ksp(lead(II) chromate) = [Pb²⁺][CrO₄²⁻]. [Pb²⁺] = √2,8·10⁻¹³ = 5,3·10⁻⁷ M.
Ksp(cobalt(II) hydroxide) = [Co²⁺][OH⁻]². [Co²⁺] = ∛1,3·10⁻¹⁵ = 1,1·10⁻⁵ M.
Ksp(cobalt(II) sulfide) = [Co²⁺][S²⁻]. [Co²⁺] = √5·10⁻²² = 2,23·10⁻¹¹ M.
Ksp(chromium(III) hydroxide) = [Cr³⁺][OH⁻]³. [Cr³⁺] = √(√1,6·10⁻³⁰) = 3,6·10⁻⁸.
Ksp(silver sulfide) = [Ag⁺]²[S²⁻].