Southampton only ask for a C in Chemistry for their graduate entry program so as long as you meet this cut off then they shouldn't look at it any further. However, don't they ask for A Level Biology if you only have Chemistry AS? Or were you planning on sitting the A2 next year? (sorry if they've changed this requirement, I only know about 2016 entry). You have more than four choices so if you hadn't already started the course then I would probably recommend just not applying to Southampton but since you've already spent the time and money then personally I would take the test and hope for the best, that's what I'm doing this year. Doing Chemistry by distance learning is difficult, I've found it really hard this year, but you've still got time to try and get a decent grade. However, if there is something else going on that means this will be particularly difficult stressful or you think you'll struggle to get a C then it's perfectly fine to drop it, you know your situation better than we do. If you would like any specific help with AS level Chemistry or the whole distance learning mess then let me know, I will be happy to attempt haha.