What type of mercury builds up in fish?

The correct answer is C. Methylmercury.

Methylmercury is a toxic form of mercury that accumulates in fish and other aquatic organisms. It forms when inorganic mercury is transformed by bacteria in the water. Methylmercury is highly bioavailable and can build up in the food chain, particularly in larger, longer-lived fish. This is why it poses significant health risks to humans and wildlife that consume contaminated fish.