Which nitrogenous base is found in RNA but not in DNA?

Uracil is the nitrogenous base that is found in RNA and not in DNA. In the structure of RNA, uracil replaces thymine, which is present in DNA. Both RNA and DNA contain cytosine and adenine, so those options do not fit the criteria of being unique to RNA. Thymine's absence in RNA is significant because uracil performs a similar role as thymine does in DNA but is structurally different. Therefore, uracil serves as a key distinction between the two types of nucleic acids, emphasizing how the genetic coding mechanisms are adapted for their specific functions in biological systems.