Biomagnification and mercury concentrations in the ocean most often occur a. throughout the food chain b. in larger fish such as tuna and swordfish c. in smaller fish such as herring and mackerel d. in crustaceans such as crab, shrimp, and lobster

The correct answer is B.
In larger fish such as tuna and swordfish, the biomagnification process is most evident.
Biomagnification presents a process of concentration of heavy metals like mercury, or other pollutants being magnified through the food chain. This means that the organisms at the top of the food chain, most often apex predators have concentrations of pollutants in their bodies hundreds of times greater than organisms at the bottom of the food chain. For example, the concentration of mercury in a shrimp is 1 unit, then a smaller fish eats 100 shrimp, and has a 100x more concentration of mercury in its body. Then a tuna eats a 100 of smaller fish, having 100x100, or 10000 times more mercury in its body than the shrimp.