Of the following, which is classified as a compound?

Ammonia is classified as a compound because it consists of two different elements, nitrogen and hydrogen, chemically bonded together in a fixed ratio. In the case of ammonia (NH₃), one nitrogen atom is bonded with three hydrogen atoms, which gives it distinct chemical properties that are different from those of the individual elements. Compounds can be defined as substances formed when two or more elements are chemically combined, resulting in new characteristics that are not present in the original elements.

In contrast, aluminum, nitrogen, and sulfur are all elements from the periodic table. They exist as individual atoms and do not contain a combination of different elements, which is what defines a substance as a compound. Hence, ammonia stands out as the correct answer, as it embodies the definition of a compound through its elemental composition and the chemical bonds that connect those elements.