The correct answer is option 4, which states that heavy water in an atomic reactor is an oxide of a heavier isotope of hydrogen. Heavy water, also known as deuterium oxide (D2O), is a form of water where the hydrogen atoms are replaced by deuterium, a heavier isotope of hydrogen.

Deuterium has an extra neutron in its nucleus compared to normal hydrogen, giving it a slightly higher atomic weight. This replacement of hydrogen with deuterium results in the formation of heavy water.

Heavy water is used in some types of nuclear reactors as a moderator, which slows down the speed of neutrons generated during the nuclear fission process. This is important for sustaining a controlled chain reaction in the reactor.

Option 1 is incorrect because deionized water does not contain deuterium and is not considered heavy water. Option 2 is incorrect because heavy water is not an oxide of oxygen, but rather it is an oxide of hydrogen. Option 3 is incorrect because heavy water is not a mixture of ice and water, but rather it is a specific form of water where the hydrogen atoms are replaced with deuterium.

In conclusion, heavy water in an atomic reactor is an oxide of a heavier isotope of hydrogen,