I'm having a difficult time on this !

To find the theorem that proves a conclusion, you need to look at the given information that makes the conclusion true. It may help to first just look at triangle WAS. We are given angles W and S, as well as side WA. Since we have two angles and one side, we can rule out SSS and SAS because each of these choices require two or more sides. 

Then, since side WA does not lie within the two given angles, the answer must be AAS. I like to think of it like this: if you are using the theorem ASA, then you have two angles and the side in between them (the arrangement of the letters is important). Choice ASA is incorrect in this case because we are not given side WS.

I hope this helps. Let me know if you need any more help.