When an appropriate ligand is added to a saturated solution of an insoluble salt, a complex ion forms and mulitple choice below?

The answer: D: the amount of the insoluble solid that can be dissolved increases.

This is because the ligand would form a soluble complex with the metal ion of the insoluble salt, and thus more substance can be dissolved.

For example, when ammonia is added to a saturated solution of copper(II) carbonate with some solid copper(II) carbonate, the solid copper(II) carbonate would be dissolved due to the formation of soluble complex ion, tetraamminecopper(II) ion.
CuCO₃(s) + 4NH₃(aq) → [Cu(NH₃)₄]²⁺(aq) + CO₃²⁻(aq)