The correct answer is option 4: An element.

Diamond is a precious gemstone that is widely used in jewelry. It is the hardest known natural material and is composed of a single element, carbon. Each carbon atom in a diamond is bonded to four other carbon atoms in a tetrahedral crystal lattice structure. The strong covalent bonds between the carbon atoms give diamond its exceptional hardness and strength.

Option 1: A compound - A compound is a substance made up of two or more different elements. Since diamond is composed of only carbon, it is not a compound.

Option 2: A metal - Metals are usually characterized by their high electrical conductivity and metallic luster. Diamond does not possess these properties, so it is not a metal.

Option 3: A mixture of compounds - A mixture is made up of two or more substances that are physically combined. Since diamond is composed of a single element, it cannot be considered a mixture of compounds.

Therefore, the correct answer is option 4: An element.