Even though the answer in the back of the book states that this correct answer is C (the nucleotide) technically, ATP is a modified nucleotide and could also be considered a correct answer.
Work Step by Step
Nucleotides are found with three phosphate groups attached to them. For example, guanine nucleotides are naturally found as GTP (guanine triphosphate). As a phosphate group breaks off to make GTP become GDP (guanine diphosphate) energy is released and this energy is used to construct the new strand of DNA. The same process occurs with all nucleotides (Adenine is ATP, Thymine is TTP, Cytosine is CTP).