As DDT passes into food chain, its concentration (a) remains same (b) decreases (c) becomes zero (d) increases.

The introduction of DDT, short for Dichloro-Diphenyl-Trichloroethane, into the food chain has been a significant environmental concern. DDT is an organochlorine pesticide that was widely used to combat diseases like malaria, by targeting the mosquitoes that spread such diseases. However, when DDT is released into the environment, it can be absorbed by plants and plankton, starting its journey through the food chain.

As DDT is fat-soluble, it tends to accumulate in the fatty tissues of organisms. Small animals and insects eat the contaminated plants, and in turn, are eaten by larger predators. With each step up the food chain, the concentration of DDT increases because each predator consumes multiple prey, which each contain their own load of DDT. This concentration effect is more readily seen as we look towards the top predators, such as birds of prey, which may suffer from severe reproductive issues due to the high levels of DDT.

Bioaccumulation refers to the buildup of chemicals, such as DDT, in an organism's body over time. This process occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost through catabolism and excretion. Since organisms at the bottom of the food chain consistently absorb DDT from their environment, they initiate the bioaccumulation process.

For instance, plankton absorbs DDT from contaminated water; small fish then consume the plankton, and larger fish eat the small fish. If the larger fish are caught and consumed by humans, the DDT can ultimately enter the human body. It's crucial to understand that bioaccumulation is a continual process, and the concentration of DDT can reach dangerous levels, especially in long-lived species and top predators, including humans.

Trophic levels delineate the positions that organisms hold within a food chain, ranging from producers at the base to apex predators at the top. These levels are pivotal to understanding how energy and nutrients flow through an ecosystem and are also instrumental in examining the biomagnification of substances like DDT.

Primary Producers: At the first trophic level, we find autotrophs like plants and phytoplankton that make their own food using sunlight or chemical energy.

Primary Consumers: These are herbivores that feed on primary producers and constitute the second trophic level.

Secondary Consumers: Carnivores that eat herbivores fall into this category, making up the third trophic level.

Tertiary Consumers: These are predators that eat secondary consumers and are on the fourth trophic level; they often face the highest risk of detrimental effects from bioaccumulation, owing to their long lifespan and dietary habits, which includes consuming multiple secondary consumers that have already accumulated DDT.