The correct order of solubility is option 'D' which is CaCO3, NaHCO3, KHCO3. Let's understand the solubility of each compound and why they are ordered in this way.

1. Calcium Carbonate (CaCO3):
- Calcium carbonate is an insoluble compound in water. It does not dissolve easily in water due to its strong ionic bond between calcium and carbonate ions.
- When calcium carbonate is added to water, it forms a white precipitate (a solid that settles at the bottom of the solution) due to its low solubility.
- Therefore, CaCO3 is the least soluble among the three compounds.

2. Sodium Bicarbonate (NaHCO3):
- Sodium bicarbonate is a moderately soluble compound in water. It dissolves readily in water to form a clear solution.
- As it dissolves, it releases sodium ions (Na+) and bicarbonate ions (HCO3-) into the solution.
- The solubility of sodium bicarbonate is higher than calcium carbonate but lower than potassium bicarbonate.

3. Potassium Bicarbonate (KHCO3):
- Potassium bicarbonate is the most soluble compound among the three. It is highly soluble in water.
- When added to water, potassium bicarbonate completely dissociates into potassium ions (K+) and bicarbonate ions (HCO3-).
- The high solubility of potassium bicarbonate makes it the most soluble compound in the given options.

To summarize, the correct order of solubility is CaCO3 (least soluble) < nahco3="" (moderately="" soluble)="" />< khco3="" (most="" />

This order is determined by the strength of the ionic bonds between the ions in each compound. Compounds with stronger ionic bonds are less likely to dissolve in water and are considered less soluble. On the other hand, compounds with weaker ionic bonds are more likely to dissolve in water and are considered more soluble.