Which of the following is a common compound?

Water (H2O) is a common compound due to its abundance and significance in everyday life and various chemical processes. It consists of two hydrogen atoms covalently bonded to one oxygen atom, forming a molecule that is essential for all known forms of life. Water serves as a solvent, participates in biochemical reactions, and plays a crucial role in temperature regulation, among other functions.

In contrast, iron, oxygen, and gold are elements rather than compounds. While iron and gold are metals with distinct properties and uses, they do not exist as compounds themselves unless combined with other elements in specific chemical forms. Oxygen exists predominantly as a diatomic molecule (O2), which is a form of the element rather than a compound since it consists of only one type of atom. Therefore, water stands out as the only compound in the list, characterized by a specific arrangement and combination of different atoms.