The correct answer is option 'A': Hardness.

Explanation:
Eriochrome Black T is a commonly used indicator for determining water hardness. It is a synthetic dye that forms a colored complex with metal ions, particularly calcium and magnesium ions. The color change occurs when Eriochrome Black T binds to these metal ions, allowing for easy detection and quantification of hardness.

Testing water for hardness is important because it helps determine the amount of dissolved calcium and magnesium ions present. High levels of hardness can lead to scale formation in pipes and appliances, reduced efficiency of water heaters, and increased soap consumption. Thus, it is essential to monitor and control water hardness in order to avoid these issues.

There are different methods to measure water hardness, and one of them involves the use of Eriochrome Black T as an indicator. This method is known as the complexometric titration method.

Complexometric titration is a type of volumetric analysis where a known concentration of a titrant (in this case, a solution of a chelating agent such as ethylenediaminetetraacetic acid (EDTA)) is added to the water sample until the endpoint is reached. The endpoint is determined by the color change of the indicator, which occurs when all the calcium and magnesium ions are complexed with the chelating agent.

Here is how the complexometric titration method using Eriochrome Black T as an indicator works:

1. A water sample is collected and filtered to remove any suspended solids.
2. A known volume of the water sample is transferred into a titration flask.
3. A few drops of Eriochrome Black T indicator are added to the flask. The indicator forms a wine-red complex with calcium and magnesium ions.
4. A standardized solution of EDTA (titrant) is slowly added to the flask while swirling the solution. EDTA acts as a chelating agent, forming stable complexes with calcium and magnesium ions.
5. The titration is continued until the color of the solution changes from wine-red to blue. This color change indicates that all the calcium and magnesium ions have been complexed with EDTA.
6. The volume of EDTA solution required to reach the endpoint is recorded.
7. The hardness of the water sample is calculated based on the volume of EDTA solution used and the concentration of the EDTA solution.

In summary, Eriochrome Black T is used as an indicator in the complexometric titration method to determine water hardness. It forms a colored complex with calcium and magnesium ions, allowing for the endpoint of the titration to be easily detected.