The correct answer is option 'B' - Chlorination.

Explanation:
Sewage treatment is the process of removing contaminants from wastewater before it is released back into the environment. It consists of several stages or processes to ensure that the water is clean and safe for disposal or reuse. The last stage in the treatment of sewage is chlorination.

1. Primary Treatment:
The first stage of sewage treatment is called primary treatment. It involves the physical removal of large solids and debris from the wastewater through processes such as screening and sedimentation.

2. Secondary Treatment:
After primary treatment, the wastewater undergoes secondary treatment. This stage involves the biological removal of organic matter and nutrients through the use of microorganisms. Various processes like activated sludge process, trickling filters, or rotating biological contactors are employed.

3. Tertiary Treatment:
Following secondary treatment, the wastewater may undergo tertiary treatment if required. This stage is aimed at further improving the quality of the effluent. Tertiary treatment methods include filtration, disinfection, and advanced oxidation processes.

4. Chlorination:
The final stage in the treatment of sewage is chlorination. Chlorination is a disinfection process that involves the addition of chlorine or chlorine compounds to the treated wastewater. It is done to kill or deactivate any remaining harmful bacteria, viruses, or other pathogens that may be present in the effluent.

Chlorine is a powerful disinfectant that can effectively destroy disease-causing microorganisms. It is added in small amounts to ensure that the water is safe for disposal or reuse. Chlorination helps in preventing the spread of waterborne diseases and protects public health.

In conclusion, the last stage in the treatment of sewage is chlorination. It is an essential step to ensure the safety and quality of the treated wastewater before it is released back into the environment.

In chlorination all the bacteria and germs are purified from water and it is release in water bodies.
Before chlorination water is fully treated with other forms so it is the last step.