Answer:
The correct treatment required before disposing sewage effluent into the river is Sedimentation and Screening.

Reason:

1. Dilution Factor:
The dilution factor refers to the ratio of the volume of effluent to the volume of receiving water. In this case, the dilution factor required is 200. This means that the volume of effluent needs to be diluted by a factor of 200 before it is discharged into the river.

2. Plain Sedimentation:
Plain sedimentation refers to the process of allowing solid particles to settle at the bottom of a tank or basin. While sedimentation can remove some suspended solids, it is not sufficient to achieve the required dilution factor of 200. Sedimentation alone does not provide enough treatment to meet the desired level of water quality.

3. Screening:
Screening is a process that involves the removal of large debris and solid particles from the effluent. It typically involves the use of screens or grates to trap and separate solid materials such as sticks, stones, and plastics. Screening is an important initial step in wastewater treatment as it prevents these solids from clogging pipes, pumps, and other equipment downstream.

4. Sedimentation and Screening:
The combination of sedimentation and screening is required to achieve the necessary treatment level before discharging the sewage effluent into the river. Sedimentation removes some suspended solids through the settling process, while screening removes larger debris. This combined treatment process helps to reduce the organic and inorganic pollutants in the effluent, improving its quality and reducing the risk of environmental pollution.

5. Complete Treatment:
Complete treatment, which involves processes such as biological treatment, disinfection, and advanced filtration, may be required for sewage effluent that needs to meet stricter water quality standards. However, in this case, where the only requirement is to achieve a dilution factor of 200, sedimentation and screening are sufficient.

Conclusion:
In conclusion, the correct treatment required before disposing sewage effluent into the river is sedimentation and screening. This combination of processes helps to remove solid particles and debris from the effluent, improving its quality and achieving the necessary dilution factor.