I think that clarification can be offered here.  Prior to the outset, I have to stress that seeking insight on a particular question is worthwhile, but the final answer for any application question has to come from one's own experience and sense of understanding.  In my mind, I think that the question is seeking to find out which particular area in your proposed field of study represents your own passion or area of significant accomplishment.  I certainly don't think that they are expecting you to have written a dissertation or something, but rather I think that you are supposed to comment on where you have shown potential promise in your proposed field of study.  In your case, what endeavors have you taken in computer science that might serve to increase your passion for continuing your study in it?  The term "master" has to be navigated here.  If you have written a particular program or contributed to a particular computer science design, then I think that delving into this might be a good thing.  Perhaps the larger issue here is what is it about computer science has you thinking of taking this as a major?  The application is assessing your ability to write about yourself in delving into who you are.  The answer might be secondary to this.  They are not looking for you to be a chief program designer at Microsoft, or having patented the new structural designs for the Apple 5 iPhone.  Rather, they want to know where your focus might lie in computer design.  Reflection and rumination might be a good start in this process.