a. College students have a part-time job in retail. (this suggests that they have one job) it i more likely that one student has one job. So I suggest that while this might be correct depending on the structure around it is less likely.
b. College students have part-time jobs in retail. This is far clearer and more correct.
in retail is great, in the retail business is great, in retail business is less good. However, let's look at another phrase
I have great experience in retail, I have great experience in the retail business. Yes, that confirms it "in retail business" is less good.