Identify the formula for sulfur trioxide.

The formula for sulfur trioxide is represented as SO3. This designation is derived from the systematic naming conventions for chemical compounds, particularly involving covalent compounds, where prefixes are used to indicate the number of atoms of each element present in the molecule.

In this compound, 'sulfur' indicates the presence of a sulfur atom, and the prefix "tri-" in "trioxide" indicates that there are three oxygen atoms associated with each sulfur atom. Thus, to formulate sulfur trioxide, one sulfur atom (S) is combined with three oxygen atoms (O).

The other formulas provided do not accurately correspond to sulfur trioxide. For instance, SO2 represents sulfur dioxide, which consists of one sulfur atom and two oxygen atoms. SO4 suggests a hypothetical molecule with four oxygen atoms, which is not the correct representation for this compound. Lastly, S3O is an unusual and non-standard formulation that suggests three sulfur atoms and one oxygen atom, which does not align with the nomenclature for sulfur trioxide. Therefore, SO3 is the correct representation reflecting the composition and stoichiometry of sulfur trioxide.