The correct answer is option 3: Fish.

Mercury is a toxic heavy metal that can be found in the environment, including water bodies like oceans and rivers. It enters the food chain through a process called biomagnification, where smaller organisms absorb the mercury and are then consumed by larger organisms.

Fish, particularly larger predatory fish, are at the top of the food chain and tend to accumulate higher levels of mercury as they consume smaller fish. This is why fish are the most likely food through which mercury enters our bodies and accumulates as a toxin.

Broiler, option 1, is likely an incorrect answer as it refers to a type of chicken meat, which does not typically have high levels of mercury.

Butter, option 2, is also an unlikely source of mercury as it is derived from milk and not known to accumulate toxins like mercury.

Spinach, option 4, is a vegetable and is not known to accumulate high levels of mercury. It is not a significant source of mercury exposure compared to fish.

Therefore, the correct answer is option 3.