Explanation:

The strength of interparticle forces generally determines the state of matter at a given temperature. Solids have the strongest interparticle forces, followed by liquids, and then gases, which have the weakest. At room temperature: Option A, Oxygen (O$_2$), is a gas, indicating very weak intermolecular forces. Option B, Water (H$_2$O), is a liquid, with stronger intermolecular forces (hydrogen bonding) than oxygen, but weaker than solids. Option C, Bromine (Br$_2$), is a liquid, with stronger van der Waals forces than oxygen but still weaker than metallic bonds. Option D, Iron (Fe), is a solid, characterized by strong metallic bonds between its atoms. Therefore, iron has the strongest interparticle forces among the given options.