**Explanation:**

**BOD (Biochemical Oxygen Demand):**
Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen required by microorganisms to decompose organic matter in water. It is an important parameter for assessing water quality and the level of pollution in water bodies. The higher the BOD value, the higher the organic pollution in the water.

**Treatment of Polluted Water:**
To treat polluted water, various treatment methods are available depending on the level of pollution and the desired water quality. The options mentioned in the question are sedimentation tanks, oxidation ponds, sludge digesters, and clarifiers.

**Sedimentation Tanks:**
Sedimentation tanks, also known as settling tanks or sedimentation basins, are used to remove suspended solids from the water by the process of sedimentation. In these tanks, the water is allowed to settle, and the solids settle at the bottom, forming a sludge layer. Sedimentation tanks are effective in removing larger particles but may not be sufficient for treating water with low BOD levels.

**Oxidation Ponds:**
Oxidation ponds, also known as stabilization ponds or lagoons, are shallow, man-made bodies of water designed to treat wastewater. These ponds rely on the natural processes of algae growth and sunlight to treat the water. Algae in the ponds consume organic matter, including BOD, through photosynthesis. Oxidation ponds are effective in treating water with low BOD levels, as the algae can efficiently remove the remaining organic matter.

**Sludge Digesters:**
Sludge digesters, also known as anaerobic digesters or biogas digesters, are used to treat the sludge generated during the wastewater treatment process. They operate under anaerobic conditions, breaking down organic matter in the sludge and producing biogas. Sludge digesters are not specifically designed for treating polluted water with low BOD levels.

**Clarifiers:**
Clarifiers, also known as sedimentation tanks or clarifying tanks, are similar to sedimentation tanks in their purpose. They are used to remove suspended solids from water by the process of sedimentation. However, clarifiers are typically used in larger water treatment plants and are more effective in treating water with higher BOD levels.

**Conclusion:**
Among the given options, oxidation ponds are the most economically suitable for treating polluted water with low BOD levels. Oxidation ponds utilize natural processes and do not require sophisticated equipment, making them cost-effective for treating water with low organic pollution levels.