According to the images that we are given, we know that:
a) We have 4 molecules of SO₃.
b) We have 2 molecules of SO₂ and 2 molecules of SO₃.
c) We have 1 molecule of O₂, 3 molecules of SO₂ and 2 molecules of SO₃.
d) We have 2 molecules of O₂ and 3 molecules of SO₂.
The reaction is:
2 SO₂ (g) + O₂ (g) -----> 2 SO₃ (g)
And the problem asks: Which diagram will result if 4 moles SO₂ reacts with 2 moles O₂?
We can read the reaction like this:
"If two molecules of SO₂ reacts with one molecule of O₂, two molecules of SO₃ are produced."
We can do something similar with moles:
"If two moles of SO₂ reacts with one mol of O₂, two moles of SO₃ are produced."
How many moles reacted according to the problem? 4 moles of SO₂ reacts with 2 moles of O₂. So 4 moles of SO₃ are produced.
Finally the diagram that represents our situation is picture A.