Which of the following statements is/are correct?

Both statements (a) and (b) are correct.

(a) The Biochemical Oxygen Demand (BOD) represents the amount of dissolved oxygen that would be consumed if all the organic matter in 1 L of water were oxidized by microorganisms: This statement is correct. BOD is a measure of the amount of oxygen required by microorganisms to decompose organic matter in water through aerobic biological processes. It is commonly used as an indicator of the organic pollution level in water bodies. Higher BOD values indicate a greater amount of organic pollutants present in the water, which leads to increased oxygen consumption by microorganisms during decomposition.

(b) Sewage water is treated to reduce its BOD: This statement is also correct. Sewage water contains high levels of organic pollutants, which contribute to a high BOD value. To mitigate the environmental impact of sewage discharge, sewage treatment plants employ various processes to reduce the BOD of sewage water. These treatment processes typically involve physical, biological, and chemical methods to remove or degrade organic matter, reducing the oxygen demand and improving the quality of treated water before its release into the environment.

(c) A high value of BOD means the water is less polluted by organic matter: This statement is incorrect. A high BOD value actually indicates that the water is more polluted by organic matter. When there is a significant amount of organic pollutants present in the water, microorganisms require more oxygen to decompose and break down this organic matter. As a result, the BOD value increases, reflecting the higher oxygen demand and the level of pollution in the water.

(d) Both statement (a) and (b) are correct: Based on the explanations provided, it can be concluded that both statements (a) and (b) are correct. BOD is a measure of the oxygen demand by microorganisms to decompose organic matter, and sewage water is treated to decrease its BOD and reduce the organic pollution before discharging it into the environment.