Final answer:
Calcium ethanedioate is sparingly soluble in water due to electrostatic forces and quite soluble in dilute hydrochloric acid due to the formation of a complex ion.
Step-by-step explanation:
The solubility of calcium ethanedioate in water is sparingly soluble due to the electrostatic forces between the cations (Ca2+) and anions (C2O4 2-) being significantly greater than the solvation forces. This makes the dissolution process endothermic and the compound does not dissolve to an appreciable extent.
However, calcium ethanedioate (also known as calcium oxalate) is quite soluble in dilute hydrochloric acid. This is because the presence of chloride ions (Cl-) in the hydrochloric acid solution forms a complex ion, CaCl2, which increases the solubility of calcium ethanedioate.
Therefore, the correct answer is c) Formation of a complex ion.