Detention period refers to the average time that wastewater spends in an oxidation pond or any other treatment system. It is an important parameter in the design and operation of wastewater treatment systems as it determines the level of treatment achieved. The correct answer to the given question is option 'C', which states that the detention period of an oxidation pond is 10-20 days.

Oxidation Pond:
- An oxidation pond is a type of wastewater treatment system that uses natural processes to treat wastewater.
- It consists of a shallow basin or series of basins where wastewater is stored and treated.
- The pond is designed to promote the growth of microorganisms that consume organic matter and nutrients present in the wastewater.
- As the wastewater flows through the pond, it undergoes various physical, chemical, and biological processes that result in the removal of contaminants.

Detention Period in Oxidation Pond:
- The detention period in an oxidation pond is the average time required for the wastewater to travel through the pond.
- It is calculated by dividing the volume of the pond by the inflow rate of wastewater.
- The detention period is an important design parameter as it determines the level of treatment achieved by the pond.
- A longer detention period allows more time for the microorganisms to metabolize the organic matter and nutrients present in the wastewater, resulting in a higher level of treatment.
- On the other hand, a shorter detention period may not provide sufficient time for the treatment processes to occur, leading to inadequate treatment.

Correct Answer Explanation:
- Option 'A' (12-36 hours) is incorrect because this detention period is more suitable for facultative ponds, which operate at a higher loading rate and provide less treatment compared to oxidation ponds.
- Option 'B' (4 hours) is incorrect because this detention period is too short for an oxidation pond to achieve effective treatment.
- Option 'D' (30-60 seconds) is incorrect because this detention period is extremely short and not applicable to oxidation ponds.
- Option 'C' (10-20 days) is the correct answer as it represents a reasonable detention period for an oxidation pond to achieve effective treatment.
- During this period, the wastewater is given sufficient time for the microbial processes to occur, resulting in the removal of organic matter and nutrients.

In conclusion, the detention period of an oxidation pond is typically 10-20 days, which allows for effective treatment of wastewater by promoting the growth of microorganisms and facilitating the removal of contaminants.