Which of the following is an example of a compound?

Water (H2O) is an example of a compound because it consists of two or more different elements chemically combined in a fixed ratio. In the case of water, it is made up of two hydrogen atoms bonded to one oxygen atom. This combination creates new properties that are distinct from the individual elements, illustrating the concept of a compound.

The other options represent either elements or molecular forms of the same element. Oxygen (O2) is a diatomic molecule made up of two oxygen atoms, but it is still classified as an element because it consists of only one type of atom. Sodium (Na) and carbon (C) are both pure elements themselves and do not combine with any other elements in this form. Thus, water stands out as the correct example of a compound in this context.