What is the key ingredient found in 2/3 of the U.S. water supply that aids in tooth and bone formation?

Fluoride is the key ingredient recognized for its significant role in the formation of teeth and bones. It enhances the re-mineralization process of tooth enamel, which can help protect against decay and cavities. According to various health and dental organizations, fluoride is not only naturally found in varying concentrations in water but is also added to many public water supplies to improve dental health on a community-wide basis. The relationship between fluoride and dental health is well-established, with studies demonstrating that communities with fluoridated water typically see lower rates of tooth decay.

In the context of the other options, while chlorine is often used to disinfect water and sodium is relevant for various bodily functions, they do not contribute to the structural integrity of teeth and bones. Calcium, though crucial for bone health, does not specifically refer to the compound added to water supplies for the purpose of dental health like fluoride does. Therefore, the unique contribution of fluoride to tooth and bone formation makes it the correct answer.