The representative partice of Ag is an atom, the representative particle of S20 is a molecule, the representative particle of O2 is a molecule and the representative particle of Fe is an atom. A representative particle is a single particle of a substance in the form in which is naturally occurs. Elements are generally atoms, with the exceptions being those such as oxygen which occur in nature as molecules. Compounds exist as molecules.
You can determine the answer to this question from the formula for each substance. Fe and Ag, for example, have no other elements included and no subscripts, representing single atoms. O2, while it only has one element, has the subscript 2 indicating the it occurs in molecules of two atoms. S2O has both a subscript and more than one atom, so you can be sure that it's a molecule.