Which of the following nitrogen bases is NOT found in RNA?

Thymine is not found in RNA; instead, RNA contains uracil in its place. In DNA, thymine pairs with adenine, but RNA utilizes uracil to pair with adenine. This difference is a key characteristic that distinguishes RNA from DNA. The presence of uracil in RNA, as opposed to thymine, is crucial for various biological processes, including protein synthesis. This understanding of the nitrogen bases is fundamental in recognizing the structural differences between DNA and RNA, which are pivotal to molecular biology and genetics.