What is B.O.D.?
A. The amount of \[{{O}_{2}}\] utilized by organisms in the water.
B. The amount of \[{{O}_{2}}\] utilized by microorganisms for decomposition.
C. The total amount of \[{{O}_{2}}\] present in water.
D. All of the above.

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Hint: The aquatic life requires oxygen to respire. They depend on the dissolved oxygen present in the water. The rest of the oxygen is used in the breakdown of organic matter. The oxygen levels required for this purpose are called biochemical oxygen demand or B.O.D.

Complete answer:The water bodies consist of a wide range of aquatic life ranging from large fishes to tiny phytoplankton and other microbes. Generally, all of the life forms require oxygen to respire and this is provided by diffusion of atmospheric oxygen into water bodies. This dissolved oxygen serves for the purpose of sustainability of marine life. Now, the cycle of life and death continues at a normal pace in water. The organic matter needs to be broken down to clean the water. The Biochemical oxygen demand describes how much of the oxygen is required to do this activity in water.
The key decomposers in water bodies are aerobic microbes. They require oxygen to break down the complex organic matter into simpler ones and then use it as their energy. The B.O.D. gives a measure for the total dissolved oxygen levels utilized by microbes in the biological metabolizing of organic molecules in water. Thus, B.O.D. denotes the amount of \[{{O}_{2}}\] required by microorganisms for the decomposition process. The water bodies with more organic matter to be decayed will have higher B.O.D.
The total amount of \[{{O}_{2}}\] present in water represents the dissolved oxygen or D.O.
So, option B is correct.

Note: The toxic streams from factories and fertilizer runoff from fields introduce an excess of nutrients into water bodies. This invites the algal species to flourish. The algal species forms a sheet over the water called the algal bloom. This inhibits the oxygen from the atmosphere to get dissolved in water. This may result in the suffocation of marine life. This phenomenon is called eutrophication.