Step-by-step explanation:
In eukaryotic cells, pre-mRNA has to undergo 3 stages of RNA processing before being transported into the cytosol. Specifically during RNA Splicing, small nuclear RNAs (snRNAs), small nuclear ribonucleoproteins (snRNPs) and other proteins form a spliceosome which excises introns and splice together exons. Introns are considered junk material and does not stay with the mRNA.
The answer is Introns. (D)