Final answer:
The reaction FeO + SO₃ → FeSO₄ is a combination or synthesis reaction, where two substances combine to form a single product, iron(II) sulfate.
Step-by-step explanation:
The reaction FeO + SO₃ → FeSO₄ is categorized as a combination reaction, also known commonly as a synthesis reaction. In this type of reaction, two or more substances combine to form a single new substance. With iron(II) oxide (FeO) combining with sulfur trioxide (SO₃) to form iron(II) sulfate (FeSO₄), we can see that there are no elements or compounds that are breaking down (which would constitute a decomposition reaction), no atoms being replaced (which would be a single replacement reaction), and no exchange of ions between two compounds (which typifies a double displacement reaction).