The correct answer is (d), oxygen.
Oxygen is essential for aquatic life, and its presence in water is a good indicator of the overall health of the ecosystem. Low levels of oxygen can be caused by pollution, such as sewage and industrial waste, as well as by natural factors such as algae blooms.
Chlorine is a disinfectant that is often used to treat drinking water. It can also be used to control algae growth in rivers and lakes. However, high levels of chlorine can be harmful to aquatic life.
Nitrogen is an essential nutrient for plants and animals. However, too much nitrogen can cause problems, such as algae blooms and the growth of invasive species.
Ozone is a gas that is formed when oxygen molecules are split by ultraviolet light. It can be harmful to aquatic life, and it can also damage human lungs.
Therefore, the best way to determine the water pollution in a river is to measure the dissolved amount of oxygen.