Which of the following describes the final product of mRNA before it exits the nucleus?

The final product of mRNA before it exits the nucleus is mature mRNA after processing. During the transcription process, the initial RNA transcript (pre-mRNA) undergoes several modifications, which include capping, polyadenylation, and splicing.

Capping involves adding a methylated guanine nucleotide to the 5' end of the RNA molecule, which protects the mRNA from degradation and assists in ribosome binding during translation. Polyadenylation adds a series of adenine nucleotides to the 3' end, further stabilizing the mRNA and facilitating its transport out of the nucleus. Splicing removes non-coding sequences called introns and joins the coding sequences known as exons together, resulting in a continuous sequence that can be translated into a protein.

Only after these modifications are complete can mRNA be considered mature and exported from the nucleus to the cytoplasm, where it will be translated into a protein. This process ensures that the mRNA is fully functional and ready for the subsequent steps of protein synthesis.