The correct answer is A. highly polluted.
BOD stands for Biochemical Oxygen Demand. It is a measure of the amount of oxygen that is used up by microorganisms in a given amount of water. A high BOD value indicates that there is a lot of organic matter in the water, which can be caused by pollution. Organic matter can come from sewage, industrial waste, or agricultural runoff. When organic matter decomposes, it uses up oxygen in the water. This can lead to a decrease in the amount of oxygen available for fish and other aquatic life, and can also make the water unsafe for human contact.
Option B is incorrect because a river with a high BOD value is not highly clean. In fact, it is highly polluted.
Option C is incorrect because a river with a high BOD value is not highly productive. In fact, it is highly polluted, and this can lead to a decrease in the amount of aquatic life in the river.
Option D is the correct answer because a river with a high BOD value is highly polluted.