Question

If a water body have amount of DDT it’s ‘biochemical oxygen demand’ (B.O.D.) will
a.- Decrease
b.- Not affected
c.- Increase
d.- First decrease then increase

Answer 1

Hint: DDT is an agricultural purpose insecticide. Biological Oxygen Demand (BOD) is the most common measure of pollutant organic material. Biological Oxygen Demand indicates the amount of putrescible organic matter present in water. Therefore, a low BOD is an indicator of good quality water, while a high BOD indicates polluted water.

Complete answer:
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed (or demanded) by aerobic biological organisms in water. These organisms require oxygen to break down organic material present in a given water sample at certain temperature over a specific time period. The BOD value is frequently expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 $^\circ C$. It is often used as a surrogate of the degree of organic pollution of water.
BOD level value is indirectly proportional to dissolved oxygen amount i.e. if BOD levels are high, dissolved oxygen (DO) levels decrease because the oxygen that is available in the water is being consumed by the bacteria. Since less dissolved oxygen is available in the water, fish and other aquatic organisms may not survive.
Dichlorodiphenyltrichloroethane, commonly known as DDT, originally developed as an insecticide, but it became popular for its environmental impacts.
So if we add DDT in water, water quality decreases and BOD level increases.

Hence, (c) option is correct according to the above explanation.

Note:
Just be aware about the relation of BOD, Pollution level and quality of water with respect to that. DDT is a pesticide and carcinogen too. So it imparts worse effects on water quality.