Final answer:
The nitrogenous base that is replaced by uracil in RNA is thymine. Adenine, guanine, and cytosine are found in both DNA and RNA, with thymine being unique to DNA and uracil unique to RNA.
Step-by-step explanation:
The nitrogenous base that never appears in RNA and is replaced with uracil is thymine. This means that the correct answer to the question is that thymine is the nitrogenous base found in DNA that does not appear in RNA, and uracil takes its place. In DNA, adenine pairs with thymine, while in RNA, adenine pairs with uracil. Both RNA and DNA contain adenine, guanine, and cytosine. The nitrogenous bases adenine and guanine are purines, while cytosine, thymine, and uracil are pyrimidines.