Nucleotides are composed of a sugar molecule (ribose in RNA and deoxyribose in DNA), a phosphate group and a nitrogen-containing base (which can be Adenine, Thymine, Guanine and Cytosine in the case of a DNA). Of those three parts, the only one that changes (and therefore identifies the nucleotide) is the nitrogen containing-base. Therefore, the correct answer is C. base.
