Interesting answers! I don't know how many posters on this thread are sixth form tutors or uni admissions staff but I'm thinking some certainly are not.
I am not either, btw. Here are my thoughts:
Drop her least favourite
Drop the one she doesn't need for her uni course
Drop the one with the worst grade
If there is correlation between these, all the better. Yes, sociology is not needed for any uni courses (not even sociology, as some sixth forms don't offer it), but the disparagement it is receiving here is shocking. It is her equal best grade so unless she hates it I would keep that. Research what degrees you need chemistry for - does she want to study biochemistry? If not, I would be inclined to drop that as it is her lowest grade. Tho if she loves it (and that is why a C is gutting) then it might be worth carrying on.
A B in chemistry is only ever better than an A in sociology if you are applying to a course where chemistry is essential. Otherwise, the A is better of course. What snobbery! DS2 (yr 13) did maths, biology and ... PE. He is the brightest in the PE class - not boasting as he is far from the brightest in maths. It's not an especially academic subject. But he needs AAB for his chosen course and frankly he is more likely to get A in PE than if he had done chemistry or physics. So on that basis, it was a much better choice.
To sum up, OP, we don't have enough info, but unless chemistry is a must in the future or she absolutely loves it, yes I would drop chemistry.
Btw geography counts as a science for uni admissions so if she wanted to study biology and two sciences were needed (they aren't everywhere) then she would be OK.