ANS:A - 0.015

Natural water contains approximately 0.015 percent of heavy water. Explanation: Heavy water, also known as deuterium oxide (D2O), is a form of water that contains a higher proportion of deuterium (a heavy isotope of hydrogen) compared to normal water. The chemical formula for heavy water is D2​O, indicating two deuterium atoms and one oxygen atom. The proportion of heavy water in natural water sources is very low. On average, natural water contains about 0.015 percent (or 150 parts per million) of heavy water. This means that for every million water molecules in natural water, approximately 15 molecules are heavy water molecules. Heavy water is relatively rare in natural water sources because deuterium is less abundant than normal hydrogen. Heavy water can be separated from normal water through processes such as electrolysis or fractional distillation, but it is primarily produced industrially for use in nuclear reactors, scientific research, and various industrial applications.