The correct answer is uracil.
The associations between the nucleobases take place via the complementarity of nucleobases. In the DNA, the base complements are adenine-thymine and guanine-cytosine. In the RNA, the base complements are adenine-uracil and guanine-cytosine. Thus, only DNA comprises thymine, while in RNA uracil is found in place of thymine.
RNA is a kind of nucleic acid, which performs an essential function in the flow of information from the DNA to the proteins. For the expression of the gene, information has to be conducted via RNA. Each nucleotide in RNA comprises a ribose sugar, a nitrogenous base, and a phosphate group. The nitrogenous bases in RNA comprise guanine, adenine, uracil, and cytosine, that is, thymine gets substituted with uracil.