The chemical system containing the simultaneous equilibria involves the precipitation of calcium oxalate in the presence of a weak diprotic acid, oxalic acid:
Ca2+(aq) + C2O42-(aq) ⇆ CaC2O4(s)
H2C2O4(aq) + H2O(l) ⇆ H3O+(aq) + HC2O4-(aq)
HC2O4-(aq) + H2O(l) ⇆ H3O+(aq) + C2O42-(aq))
Discuss how reactant concentrations change as equilibrium reactions shift to the left or the right. Likewise, discuss how product concentrations change as equilibrium reactions shift to the left and the right. Explain using your observations if you would prepare a calcium oxalate precipitate in acidic or basic solution.

From the equations, it is clear that addition of acid favours the precipitation of calcium oxalate. As acid is added, the equilibrium position continues to shift to the far right, favouring the complete dissociation of the oxalic acid. This decreases the pH of the solution by increasing oxonium ion concentration.

Similarly, as the oxalate ion concentration increases, the precipitation of the calcium oxalate is favoured at low pH. Hence the answer.