The correct answer is option 3, high levels of fluoride in the water. Mottling of dental enamel, also known as dental fluorosis, is caused by the excessive consumption of fluoride during tooth development. This usually occurs in children when their teeth are still developing, between the ages of 1 and 8.

Fluoride is commonly added to water supplies as a public health measure to prevent tooth decay. However, when consumed in excess, it can cause dental fluorosis. This condition manifests as white or brown stains on the teeth, as well as pitting and enamel irregularities.

Option 1, high levels of chlorine in the water, is not a cause of dental enamel mottling. Chlorine is added to water to disinfect it, but does not have a direct effect on the enamel.

Option 2, high levels of nitrate in the water, is also not a cause of dental enamel mottling. Nitrate is commonly found in water sources, but does not affect the enamel.

Option 4, high levels of calcium in the water, is unrelated to dental enamel mottling. Calcium is an essential mineral for tooth and bone health, and does not cause mottling when consumed in appropriate amounts.