D. 2S + 3O2 right-pointing arrow 2SO3
Let's look at the 4 options and see what the issues are with each of the wrong ones.
A. Sulfur and oxygen react to form sulfur trioxide.
* This is just a quick description, but nothing quantitative. How much sulfur? How much oxygen? Are the reactants single atoms or molecules? Not enough information, so this is the wrong choice.
B. S and O2 produce SO3.
* Just replacing words with element abbreviations does't fix the problem. So it's still wrong.
C. S + O2 right-pointing arrow SO3
* This is getting closer. We have the correct symbols for the reactants and product. But the equation doesn't balance. On the left hand side, there are 2 oxygen atoms and on the right hand side, there's 3. Where did the 3rd oxygen atom come from? So it's still wrong.
D. 2S + 3O2 right-pointing arrow 2SO3
* This is correct. We know that we're using elemental sulfur and an oxygen molecule on the left hand side. Actually 2 atoms of elemental sulfur and 3 molecules of oxygen gas. And we're producing 2 molecules of sulfur trioxide. And the numbers of each type of atom match in count on both sides. So this is the correct answer.
