The correct answer is D, compound. It is defined as a chemical substance, which has 2 or more bonded chemical elements. It has a fixed ratio that determines a composition. Chemical formulas are used to express the ratio of each element. There are four types of chemical compounds. The first is molecules, which are held together by covalent bonds. An example of a molecule is H20.
The second chemical compound is intemetallic, which are held together by metallic bonds. An example is Nitinol. The third is an ionic compound held by ionic bonds. An example if an ionic bond is sodium (Nat). The last and final compound is certain complexes, which are held by coordinate covalent bonds. An example is etnene.