You say the one it has on your qualifications certificate. The code should be to the left of the subject on the sheet where it says what units you've done aswell.
Oh I had that aswell, was confused for the Art and Business Management one since there were 2 to choose from. I checked my certificate for Art and asked the teacher (I was filling it out in a study period and my Business teacher takes that period so i just asked her).
I left them 'awaiting' or whatever it says. UCAS gives the uni's your results when they come next year automatically i think.