The correct answer is option C, 80.

Explanation:
Heavy water, also known as deuterium oxide (D2O), is a form of water in which the hydrogen atoms are replaced by the isotope deuterium. Deuterium is an isotope of hydrogen that contains one proton and one neutron in its nucleus, making it twice as heavy as the common hydrogen isotope, protium.

The chemical formula for heavy water is D2O, indicating that it contains two deuterium atoms and one oxygen atom. In regular water (H2O), the oxygen atom is attached to two hydrogen atoms.

To determine the percentage of oxygen in heavy water, we need to calculate the molar mass of heavy water and the molar mass of oxygen.

The molar mass of heavy water (D2O) can be calculated as follows:
2 (molar mass of deuterium) + 1 (molar mass of oxygen)
= (2 x 2.014) + 15.999
= 4.028 + 15.999
= 20.027 g/mol

The molar mass of oxygen (O) is 15.999 g/mol.

To find the percentage of oxygen in heavy water, we can use the following formula:

Percentage of oxygen = (molar mass of oxygen / molar mass of heavy water) x 100

Plugging in the values, we get:
Percentage of oxygen = (15.999 / 20.027) x 100
= 0.799 x 100
= 79.9%

Therefore, the percentage of oxygen in heavy water is approximately 80% (option C).