First of all, I need to know what these five salts are. Luckily, I found a similar problem from another website which is shown in the attached picture. The Ksp is the solubility product constant. It follows the formula:
Ksp = [cation concentration]ᵃ[anion concentration]ᵇ
where a and b are the subscripts of the metal and nonmetal, respectively. 
For the solutions ahead, let x be the concentration of the cation.
A.  The formula is PbCr₂O₄. 
2.8×10⁻¹³ = [x][x]
Solving for x, x = 5.29×10⁻⁷ M
B. The formula is Co(OH)₂. 
 1.3×10⁻¹⁵ = [x][x]²
Solving for x, x = 1.09×10⁻⁵ M
C. The formula is CoS. 
 5×10⁻²² = [x][x]
Solving for x, x = 2.24×10⁻¹¹ M
D. The formula is Cr(OH)₃. 
 1.6×10⁻³⁰ = [x][x]³
Solving for x, x = 3.56×10⁻⁶ M
E. The formula is Ag₂S. 
 6×10⁻⁵¹ = [x]²[x]
Solving for x, x = 1.82×10⁻¹⁷ M
<em>Thus,the highest concentration is letter B, Cobalt (II) Hydroxide.</em>