The correct answer is: introns
Processing of mRNA is process that occurs only in eukaryotic cells. After the transcription, newly formed mRNA molecule is called and it must be processed into a messenger RNA (mRNA). Processing includes:
• Addition of a 5' cap to the beginning of the RNA
• Polyadenilation- addition of a poly-A tail (tail of A nucleotides) to the end of the RNA
• Splicing- removal of introns and linking the exons
After these steps mRNA is mature and used for the further steps (e.g. translation).
Sometimes some genes are alternatively spliced.