Final answer:
Carbon (D) does not exist as a diatomic molecule; it forms other types of bonds, while hydrogen, oxygen, nitrogen, chlorine, fluorine, bromine, and iodine do form diatomic molecules.
Step-by-step explanation:
The element that does not exist as a diatomic molecule among the provided options is carbon (D). Diatomic molecules consist of two atoms of the same element bonded together. The elements known to exist naturally as diatomic molecules include hydrogen (H₂), oxygen (O₂), nitrogen (N₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂). Carbon typically bonds with other elements or forms chains and rings with other carbon atoms rather than existing naturally as a diatomic molecule.