Chlorination involves adding a measured amount of chlorine to water to produce a residual sufficient to kill bacteria, viruses, and cysts. The killing effect of chlorine depends on the pH of the water, temperature, chlorine level and contact time (i.e., the time the chlorine is in the water before consumption).

Chemical treatment for water purification mainly involves the process of chlorination, which is represented by option 'D'. Let's understand the different methods of water purification and why chlorination is considered an effective chemical treatment.

1. Sedimentation:
Sedimentation is a physical process that involves the settling down of heavy particles present in water due to gravity. It helps in removing large impurities and solids but does not involve any chemical treatment.

2. Decantation:
Decantation is a process of separating liquids from solids by carefully pouring off the liquid, leaving the solids at the bottom. Similar to sedimentation, it is a physical process and does not involve chemical treatment.

3. Filtration:
Filtration is a method that uses a porous medium, such as sand or activated carbon, to trap impurities and particles present in water. It can remove larger particles but may not effectively eliminate all types of contaminants. Filtration is a physical process and does not involve chemical treatment.

4. Chlorination:
Chlorination is a chemical treatment method widely used for water purification. It involves the addition of chlorine or its compounds, such as sodium hypochlorite, to disinfect water by killing bacteria, viruses, and other microorganisms. Chlorine is a strong oxidizing agent that destroys the cell walls and proteins of these harmful microorganisms, making the water safe for consumption.

Chlorination is considered an effective method of water purification due to the following reasons:

- It kills a wide range of disease-causing microorganisms, including bacteria and viruses.
- It helps in controlling the growth of algae and other waterborne organisms.
- It provides residual protection, meaning that some chlorine remains in the water even after treatment, preventing recontamination during distribution.
- It is relatively easy to implement and cost-effective compared to other methods.

Therefore, among the given options, chlorination is the method that involves chemical treatment for water purification.