Answer:
Sulfur trioxide: SO3.
Step-by-step explanation:
How I came to this conclusion:
I noticed that the compound consists of non-metals, therefore it must be a covalent compound -
meaning I use prefixes for compounds.
Sulfur is S. Simple enough. Trioxide has the prefix of tri, meaning "3". Therefore there is 3 atoms of oxygen. Combine them and I get SO3.
Don't be confused with sulfite: (SO3)2− and sulfate: (SO4)2−