The correct answer is option 3: Fluoride. Fluoride is a mineral that is essential for the development of healthy teeth. It helps to prevent tooth decay by making the enamel of the teeth more resistant to acids produced by bacteria.

However, if consumed in higher quantities, fluoride can cause a condition called dental fluorosis, which leads to the mottling of teeth. Dental fluorosis occurs when too much fluoride is ingested during the development of the teeth, causing white or brown discoloration and pitting of the enamel. This can happen if children ingest excessive amounts of fluoride toothpaste or if they consume water with high levels of naturally occurring fluoride.

It is important to note that the amount of fluoride needed for healthy teeth is relatively small, and the risk of dental fluorosis is minimal when proper amounts of fluoride are used. Dentists and health authorities often recommend the use of fluoride toothpaste and the consumption of fluoridated water to help maintain oral health, but it is important to use them in appropriate quantities.