Hydrogen bonds need to be broken as well as phosphodiester bonds. So, I would think A and C, but that isn't a choice. So, I'd go with choice E (A, B and C) and that would be the case if you also consider that phosphodiester bonds are also covalent. A little redundant I think. I don't see other covalent bonds that must be broken to produce a single nucleotide (base-sugar-phosphate).