The correct answer is option 3, high levels of fluorides in the water. Mottling of dental enamel, also known as dental fluorosis, is caused by the excessive intake of fluoride during tooth development. Fluoride is commonly found in water sources, toothpaste, and other dental products. When a person ingests too much fluoride, especially at a young age when their teeth are still developing, it can lead to mottled or discolored enamel. This condition can range from mild, with barely noticeable white spots, to severe, with brown discoloration and pitting of the enamel. It is important to note that fluorosis is a cosmetic issue and does not typically cause any functional problems with the teeth. To prevent dental fluorosis, it is recommended to monitor fluoride intake, such as using fluoridated toothpaste in moderation and drinking fluoride-free water if necessary.