The correct answer is A. increases.
DDT is a persistent organic pollutant (POP) that is highly toxic to many animals, including humans. It is not easily broken down by the environment, so it can accumulate in the bodies of animals that eat it. This is known as bioaccumulation. As DDT moves up the food chain, the concentration of DDT in the bodies of animals increases. This is because animals at higher trophic levels eat animals at lower trophic levels, and so they take in the DDT that has accumulated in the bodies of those animals. This can lead to high levels of DDT in the bodies of top predators, such as eagles and fish-eating birds. These high levels of DDT can cause reproductive problems, immune system problems, and other health problems.
Option B is incorrect because the concentration of DDT does not remain constant as it travels along the food chain. It increases.
Option C is incorrect because the concentration of DDT does not decrease as it travels along the food chain. It increases.
Option D is incorrect because the concentration of DDT does not fluctuate randomly as it travels along the food chain. It increases in a predictable way.