Okay this was the answer I got as well. Just confirming in case I point you down the wrong track
This question requires a little bit of thinking, best thing to do-start from the units required-think to yourself, how do you get mols from molecules? How do you get m^-3?
Then essentially what I did was find the number of mols of molecules in 1DU by diving molecules number by Avogadro's number.
Then you find volume by times 1m^2 by the thickness 1mm (convert this to m)-you should get 1*10^-5m^3
Essentially I then times the number of moles per molecule in 1DU by 220 to get moles in 220DU, and then times it by 10^-5 to get units in mol/m^3-you should get an answer in around 9800s which rounds up to give you D. Hope this helps