Which nucleotide is commonly repeated at the end of eukaryotic mRNA?

The correct answer is adenine. In eukaryotic mRNA, a repetitive sequence of adenine nucleotides, known as the poly-A tail, is added to the 3' end of the mRNA molecule during post-transcriptional modification. This poly-A tail serves several important functions, including enhancing the stability of the mRNA, aiding in the export of mRNA from the nucleus to the cytoplasm, and facilitating translation by promoting the recognition of the mRNA by ribosomes.

The other nucleotides mentioned do not fulfill this specific role in mRNA stabilization and modification. Uracil, cytosine, and guanine play roles in RNA synthesis and structure, but they are not involved in the formation of the poly-A tail found at the end of eukaryotic mRNA.