discrete molecules

The answer is A.

A. correct
Carbon dioxide has simple molecular structure. One carbon atom is joined with two oxygen atoms with C=O double bonds to form a carbon dioxide molecule. The O=C=O molecules are discrete and held together by weak van der Waals' forces.

B. incorrect
Diamond has giant covalent structure. In diamond, the carbon atoms are held together by a network of strong covalent bonds, and there is no discrete molecule.

C. incorrect
Sodium chloride has giant ionic structure. In sodium chloride solid, the sodium and chloride ions are held together by strong ionic bonds, and there is no discrete molecule.

D. incorrect
Iron has giant metallic structure. In iron, the iron atoms are held together by strong metallic bonds which are formed by delocalized electrons, and there is no discrete molecule.

因為iron係metallic ion而不是molecule..