Which of the following does NOT naturally contain fluoride?

Pork does not naturally contain fluoride, making it the correct answer in this context. Fluoride is primarily found in mineral sources and certain foods. Fish, especially those from marine environments, can have some levels of fluoride due to their habitat and diet, which may include algae and other sources that accumulate fluoride. Tea is also known for its fluoride content, as the leaves can absorb fluoride from the soil and water in which they are grown. Vegetables can contain trace amounts of fluoride depending on soil conditions and the use of fluoride-containing fertilizers, but they are not generally considered a significant source. In contrast, pork, being a land-based animal protein, does not accumulate fluoride to any notable extent, which is why it stands out as the option that does not naturally contain fluoride.