Which of the following elements is a diatomic molecule?

A diatomic molecule consists of two atoms, which can be the same or different, bonded together. Oxygen is a classic example of a diatomic molecule because it naturally exists as O₂ in the atmosphere, consisting of two oxygen atoms bonded together. This diatomic form is essential for various biological processes, including respiration in many organisms.

In contrast, the other options either consist of single atoms or do not occur naturally as diatomic molecules. Helium is a monoatomic gas, meaning it exists as individual He atoms. Carbon primarily exists in solid forms (such as graphite and diamond) and does not naturally form diatomic molecules under standard conditions. Iron, as a metal, comprises individual iron atoms in its solid-state and does not form molecular structures in the same manner as oxygen.

Hence, the correct answer is oxygen, as it is the only option that exists in nature as a diatomic molecule.