The model below represents a dna molecule. the presence of which element best provides evidence that nucleotides are the building material used to make dna?

DNA has 3 components; phosphate, a sugar called deoxyribose (a 5 carbon sugar as opposed to the sugar in the picture which is a 6 carbon sugar - glucose) and a nitrogenous base (adenine, thymine, guanosine or cytosine). So if you look at those 3 pictures, the only one is the nucleotide that's part of the DNA.

Re-read the question: the presence of phosphorous is the answer!!