In a double replacement reaction resulting in the formation of solid calcium carbonate, the resulting supernatant would be described as .
a) saturated with CaCO3
b) containing no ions of CaCO3
c) supersaturated with CaCO3
d) unsaturated with CaCO3
I know it's a, but I don't understand why.

a) saturated with CaCO3, because when maximum solute gets dissolve to form precipitate as when solute is saturated with solvent and then precipitates hence the supernatant is saturated with CaCO3.

Explanation:

In double displacement reaction two ionizable compounds exchange their cation or anion forming an insoluble compound that is precipitate. The supernatant will be the solvent and dissolved solute.

We know that precipitation is formed when solubility of the compound exceeds to that of other.

The solubility occurs when it exceeds the saturation in the solution.

So, the precipitate has overcome the saturation in solvent, the resulting supernatant is saturated with CaCO3 or the solute.