Bioamplification is due to:
(A) Increase in the concentration of pesticides in water
(B) Gradual accumulation of pesticides from lower animals to higher animals
(C) Gradual decrease in concentration of pesticides in $H_2O$
(D) Increase in space of biosphere

Hint: To understand the concept of bioamplification, we need to understand the concept of trophic levels and food chain. Food chain is a linear sequence of transfer of matter and energy in the form of food from one organism to another starting from producers to other higher organisms. Trophic level is a group of organisms which occupy the same level in a food chain.

Complete answer:
We have to remember that the Bioamplification, also known as biomagnification is the increase in concentration of a substance as we go higher in a food chain.
This occurs when the substance is a biologically active pollutant whose concentration increases as we go higher in a food chain.
The pollutant cannot be or broken very slowly by the organisms’ natural processes. The rate of transfer is faster than the rate of excretion.
The pollutant is mobile so that it enters into the food chain through means of water and food. If it is not mobile, it gets trapped at a single trophic level.
Let us take the help of an example of mercury bioamplification:
(i) Phytoplanktons take up methylmercury released by coal mines, volcanoes or mines and retain it in their tissues.
(ii) As other members of the next trophic level consume these phytoplanktons, they receive their
prey’s mercury share.
(iii) Members of the next higher trophic levels now will have higher levels of mercury due to constant
exposure and thus bioamplification of mercury takes place.
We now know that bioamplification is the gradual increase of a biologically active pollutant, such as a pesticide as we go higher in a food chain, i.e. from lower to higher animals.
Therefore option (B) is correct.
Bioamplification is an increase in the concentration of pesticides or any other pollutants within organisms.
Therefore option (A) is incorrect.
Bioamplification deals with an increased factor of pollutants and not decrease.
Therefore, option (C) is incorrect.
Bioamplification has no relation with space. It deals with the gradual increase of concentration of pollutants within a food chain irrespective of space.
Therefore, option (D) is incorrect.
And hence option B is correct.

Note:
There is often a confusion between bioamplification, bioaccumulation and bioconcentration. As we know bioamplification is the increase in concentration of a pollutant as we go higher in a food chain; bioaccumulation is the building up of a pollutant within an organism (does not affect the entire food chain); bio concentration is the increase in the concentration of pollutants in water (option A) and aquatic organisms adsorb these pollutants into their tissues.