The molecular structure of water contains two atoms of hydrogen and atom of oxygen. when water reaches its boiling point and turns into water vapor what happens to its molecular structure?

The correct option is this: THE MOLECULAR STRUCTURE REMAINS THE SAME.
Molecular structure refers to the type of elements and the number of the atoms of each element in a particular compound. For instance, the elements in water are oxygen and hydrogen; one atom of oxygen is present and two atoms of hydrogen[ H2O]. When water turns to vapour, the chemical force holding the atoms of the element together is broken but that does not change the amount of atoms of each element or the elements that are present in the water, thus the structure remains the same.
When the water vapour become cooled, water is formed again.