The key expression in the sentence is “hold-up.” In this context, “hold-up” means a delay or something that slows down a process. So the sentence: “I couldn't buy the biscuit because of the hold-up in the supermarket.” means that there was a delay in the supermarket which prevented the purchase. Among the options, “the attendant was slow” best explains a delay in service, which could cause such a hold-up.