Which of the following bases is found only in RNA?

Uracil is the correct answer because it is a nitrogenous base that is unique to RNA. In the structure of RNA, uracil pairs with adenine, serving a similar role as thymine does in DNA. Unlike DNA, which contains thymine, RNA replaces thymine with uracil. This difference is significant in the function and stability of the two nucleic acids.

Adenine, cytosine, and thymine are also components of DNA. Adenine and cytosine are found in both DNA and RNA, while thymine is exclusive to DNA. Since uracil is absent in DNA and is specific to RNA, it clearly differentiates the two types of nucleic acids. Understanding the unique attributes of these nitrogenous bases helps clarify the biological roles and structures of RNA and DNA.