Step 1: Set up the equilibrium table:
If $S$ mol/L dissolves, we get $2S$ of silver ions and $S$ of oxalate ions.
Step 2: Solve:
$K_{sp} = 4S^3$. Divide by 4 and take the cube root. This gives $S = (K_{sp}/4)^{1/3}$, which is option (C). The salt has the $\text{A}_2\text{B}$ type, so the factor is 4 and the root is a cube root.
Final Answer:
$S = \left(\frac{K_{sp}}{4}\right)^{1/3}$.
\[ \boxed{S = \sqrt[3]{\frac{K_{sp}}{4}}} \]