Biochemical Oxygen Demand (BOD) is the amount of oxygen required (in ppm):

First, we recall the precise definition of Biochemical Oxygen Demand, abbreviated as BOD. In environmental chemistry, BOD is that amount of dissolved oxygen which is consumed by aerobic micro-organisms while they oxidise the biodegradable organic matter present in a definite volume of a water sample, generally over a fixed time (5 days) at a fixed temperature (20 °C).

Stating the definition in mathematical form, we write

$$\text{BOD} = \frac{\text{Mass of }O_2\text{ consumed (mg)}}{\text{Volume of sample (L)}},$$

and because $$1\;\text{mg}\,\text{L}^{-1}=1\;\text{ppm},$$ the numerical value of BOD is usually expressed in $$\text{ppm}$$ (parts per million).

Now we compare each option with this definition:

Option A speaks of oxygen “for sustaining life in a water body.” That is a description of the dissolved oxygen content, not of the oxygen consumed. Hence it does not match the definition of BOD.

Option B states that oxygen is required “by bacteria to break-down organic waste in a certain volume of a water sample.” This wording exactly mirrors the definition above: aerobic bacteria (micro-organisms) oxidise organic waste, and the oxygen they consume per volume is BOD. So Option B is consistent.

Option C mentions “photochemical breakdown.” Photochemical reactions involve light, not bacteria, so this does not coincide with the biochemical process that defines BOD.

Option D refers to “anaerobic bacteria” acting on “inorganic waste.” BOD involves aerobic (oxygen-using) processes and organic matter, so Option D is also incorrect.

Only Option B satisfies all parts of the formal definition stated above.

Hence, the correct answer is Option B.