Introduction:
The biological treatment of wastewater is a process that utilizes microorganisms to break down organic matter and remove pollutants from the water. This process plays a crucial role in wastewater treatment plants as it helps in reducing the biological oxygen demand (BOD) of the water.

Explanation:
The purpose of biological treatment of wastewater is to reduce BOD. Here's why:

1. Biological Oxygen Demand (BOD):
BOD is a measure of the amount of oxygen required by microorganisms to decompose the organic matter present in wastewater. The higher the BOD, the greater the organic pollution in the water. Reducing BOD is important as high levels of organic pollutants can deplete oxygen levels in water bodies, leading to the death of aquatic organisms.

2. Role of Microorganisms:
During biological treatment, microorganisms such as bacteria and fungi are introduced to the wastewater. These microorganisms utilize the organic matter present in the water as a food source. As they metabolize the organic compounds, they consume oxygen from the water, thereby reducing the BOD.

3. Aerobic and Anaerobic Processes:
Biological treatment can occur through aerobic or anaerobic processes, depending on the availability of oxygen. In aerobic treatment, oxygen is supplied to support the growth of aerobic bacteria that require oxygen for their metabolic activity. These bacteria break down organic matter and reduce the BOD. In anaerobic treatment, microorganisms function in the absence of oxygen and convert organic matter into biogas (methane and carbon dioxide).

4. Treatment Methods:
There are various biological treatment methods used to reduce BOD in wastewater:

- Activated Sludge Process: In this process, wastewater is mixed with a culture of microorganisms called activated sludge. The sludge contains a diverse population of bacteria that consume organic matter and reduce BOD.
- Trickling Filters: Wastewater is passed over a bed of rocks or plastic media where a biofilm of microorganisms develops. These microorganisms degrade the organic pollutants and reduce BOD.
- Sequencing Batch Reactor (SBR): This process involves filling a reactor tank with wastewater, allowing microorganisms to degrade the organic matter, and then settling the solids. The treated water is then discharged, reducing the BOD.

Conclusion:
The biological treatment of wastewater is aimed at reducing the biological oxygen demand (BOD) by utilizing microorganisms to break down organic matter. This process is crucial in wastewater treatment plants to ensure the removal of organic pollutants and protect water bodies from oxygen depletion. Therefore, the correct answer is option 'A' - to reduce BOD.