The correct option is D BOD
The graph shows changes that one may notice following discharge of sewage into a river.
Micro-organisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge. This causes mortality of fish and other aquatic creatures.
A - dissolved oxygen and B - BOD.
Therefore, the correct answer is option D.