What compound group can vary in a nucleotide?
The pentose sugar or nitrogenous base
So in nucleotides, if its DNA it has deoxyribose for the pentose sugar and if its RNA it has ribose sugar.
The nitrogenous bases change, and that is what makes each organism unique. The bases are made of Cytosine, Guanine, Thymine and Adenine in DNA and Cytosine, Guanine, Adenine and Uracil in RNA