Final answer:
Option (a), Fish contain the largest levels of mercury due to biomagnification, where larger predatory fish accumulate mercury from smaller organisms they consume.
Step-by-step explanation:
The animals containing the largest levels of the nerve toxin mercury are fish. This is because, through a process known as biomagnification, mercury accumulates in the bodies of organisms over time. Larger animals, such as predatory fish, consume many smaller organisms and end up with higher concentrations of mercury in their bodies. Coal burning, which releases mercury into the environment, plays a significant role in this process.
Once mercury enters the water system, it is converted into methylmercury, which is a highly toxic form that builds up in aquatic life, especially fish that are higher up in the food chain. Therefore, when considering biomagnification of toxins, it is important to be mindful of mercury from fish consumption.