EDTA (Ethylenediaminetetraacetic acid) is a chelating agent that forms stable complexes with metal ions. It is commonly used in analytical chemistry to estimate the concentration of metal ions in a solution. In the case of EDTA titration, it is used to estimate both Ca2+ and Mg2+ ions.

Here is a detailed explanation of why the correct answer is option 'C' (both Ca2+ and Mg2+ ions):

- **Chelating Agent:** EDTA is a polydentate ligand, which means it has multiple binding sites that can form coordination complexes with metal ions. It forms a stable, water-soluble complex with metal ions by surrounding the metal ion with its electron-donating groups. This property of EDTA makes it an ideal reagent for the estimation of metal ions.

- **Complex Formation:** EDTA has a higher affinity for metal ions than water molecules due to its electron-donating groups. When EDTA is added to a solution containing Ca2+ and Mg2+ ions, it forms stable complexes with both ions. The formation constant for the Ca2+-EDTA complex is slightly higher than that of the Mg2+-EDTA complex. However, both complexes are sufficiently stable to be detected during titration.

- **Titration Process:** In EDTA titration, a known volume of the analyte solution that contains Ca2+ and Mg2+ ions is titrated with a standardized solution of EDTA. A suitable indicator is used to signal the endpoint of the titration. The most common indicator for Ca2+ and Mg2+ estimation is Eriochrome Black T (EBT), which forms a wine-red complex with metal ions.

- **Endpoint Detection:** During the titration, EDTA reacts with the metal ions present in the solution until all the metal ions have formed complexes with EDTA. At this point, the appearance of the indicator changes, indicating that the endpoint has been reached. The color change from blue to wine-red indicates the formation of the metal-EBT complex and signifies the complete complexation of both Ca2+ and Mg2+ ions with EDTA.

- **Calculation of Metal Ion Concentration:** Once the titration is complete, the volume of EDTA solution required to reach the endpoint is used to calculate the concentration of Ca2+ and Mg2+ ions in the original solution.

In conclusion, EDTA is a versatile chelating agent used in the estimation of metal ions. In the case of Ca2+ and Mg2+ estimation, EDTA forms stable complexes with both ions, allowing their concentration to be determined accurately through titration. Therefore, option 'C' (both Ca2+ and Mg2+ ions) is the correct answer.