Introduction:
Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen required by microorganisms to decompose organic matter in water. It is commonly used as an indicator of water pollution. A higher BOD value indicates a higher level of pollution in the water body. In the context of the given question, we need to determine the significance of a higher BOD in a particular segment of a river.

Explanation:
1. Biochemical Oxygen Demand (BOD) and pollution:
- BOD is a measure of the amount of oxygen consumed by microorganisms as they break down organic matter in water.
- Organic matter in water can include sewage, agricultural runoff, and other pollutants.
- When the organic matter decomposes, it consumes oxygen from the water, leading to a decrease in dissolved oxygen levels.
- Higher BOD values indicate higher levels of organic pollution in the water, as more oxygen is required for decomposition.

2. Significance of a higher BOD in a river segment:
- A higher BOD value in a particular segment of a river indicates a higher level of pollution in that segment.
- The organic pollutants can come from various sources such as sewage discharge, industrial effluents, or agricultural runoff containing fertilizers and pesticides.
- These pollutants can have detrimental effects on the water quality and aquatic life in the river segment.

3. Effects of pollution on the river:
- Higher BOD values imply a reduced amount of dissolved oxygen in the water, making it difficult for aquatic organisms to survive.
- Aquatic plants and animals require oxygen for respiration, and a decrease in oxygen availability can lead to suffocation and death.
- The presence of pollutants can also cause chemical imbalances in the water, further impacting the health and biodiversity of the river ecosystem.

4. Conclusion:
Based on the above explanations, we can conclude that a higher biochemical oxygen demand (BOD) in a particular segment of a river indicates that the segment is highly polluted (option B). The higher BOD value signifies a higher level of organic pollution, which can adversely affect the water quality and aquatic life in that segment.