A pure substance is a substance with a consistent ration of the elements that make it up. This means that there is no way to tell one water molecule apart from the other. The atoms in a pure compound contain atoms bonded in the same way throughout the compound.
On the other hand, a mixture has constituents that have different pure substances mixed together. The constituents of a mixture are easy to distinguish from each other.
With these definitions in mind, we can easily see that water is a pure sustenance because water molecules all have a consistent ratio of hydrogen to oxygen atoms.
The correct answer for this question is D)Because water molecules all have a consistent ratio of hydrogen to oxygen, it is a pure substance.