The detention period for oxidation ponds is usually kept as

In environmental engineering, the detention period for oxidation ponds is typically kept as 10 to 15 days (Option C).

Oxidation ponds, also known as stabilization ponds, are wastewater treatment systems that use natural processes such as sunlight, algae growth, and bacterial action to treat wastewater. The detention period refers to the average time that wastewater remains in the pond before being discharged.

A detention period of 10 to 15 days allows sufficient time for the biological and chemical processes in the oxidation pond to effectively treat the wastewater, reducing its pollutant levels and improving its quality before discharge.

Therefore, the correct option is Option C: 10 to 15 days.