Final answer:
Mercury is relatively high in salmon due to the process of biomagnification.
Step-by-step explanation:
The correct answer is a. salmon.
Mercury is a heavy metal that can accumulate in the bodies of long-living, predatory fish through a process called biomagnification. These fish consume smaller organisms that have absorbed mercury, leading to its build-up in their bodies over time. Therefore, fish like salmon, which are higher up the food chain and consume smaller organisms, tend to have higher levels of mercury compared to other types of fish like cod, shark, most shellfish, and tilapia, highlighting how the process of biomagnification impacts different fish species, emphasizing the elevated mercury content found in predators like salmon.