The correct answer is: C. BOD > COD > TOD
Theoretical oxygen demand (TOD) is the amount of oxygen that would be consumed if all of the organic matter in a water sample were oxidized. Biochemical oxygen demand (BOD) is the amount of oxygen that is consumed by microorganisms in a water sample over a period of time, typically five days. Chemical oxygen demand (COD) is the amount of oxygen that is consumed by a chemical oxidizing agent, such as potassium dichromate, in a water sample.
BOD is always greater than or equal to COD, because COD measures the total amount of oxidizable organic matter in a water sample, while BOD measures only the amount of organic matter that is biodegradable by microorganisms. TOD is always greater than or equal to COD, because TOD measures the total amount of organic matter in a water sample, including both biodegradable and non-biodegradable organic matter.
Therefore, the correct relation between theoretical oxygen demand (TOD), Biochemical oxygen demand (BOD) and Chemical oxygen demand (COD) is given by:
BOD > COD > TOD