Explanation:

Hawk being the last one in food chain will have the highest concentration. This is known as bio-magnification.
Biomagnification, also known as bioamplification or biological magnification, is the increasing concentration of a substance, such as a toxic chemical, in the tissues of organisms at successively higher levels in a food chain. This increase can occur as a result of:
Persistence – where the substance cannot be broken down by environmental processes
Food chain energetics – where the substance's concentration increases progressively as it moves up a food chain
Low or non-existent rate of internal degradation or excretion of the substance – often due to water-insolubility and solubility in fatty tissues which keep on rising in bigger organisms.
Another reason is that each higher organism in the food chain consumes more of lower organisms thereby leading to increased concentration.