Which of the following bases is not associated with RNA?

Thymine is the correct answer because it is a nitrogenous base that is present in DNA but not in RNA. In RNA, thymine is replaced by uracil. Both adenine and uracil are present in RNA, along with cytosine. The bases that form part of RNA’s structure include adenine, uracil, cytosine, and guanine, but thymine is exclusively found in DNA. This distinction is important in understanding the structural differences between DNA and RNA, particularly in the context of genetic information encoding and synthesis processes.