Final answer:
Only gene-specific regions of the genome, which include the exons of expressed genes, need to be transcribed into mRNA in eukaryotic cells.
Step-by-step explanation:
The genome sections that need to be transcribed are gene-specific regions, particularly the exons, which are the coding sequences of the genes. In eukaryotic cells, genes are made up of exons and introns. Exons are the regions that encode protein sequences and are expressed, whereas introns are noncoding sequences that are removed from the pre-mRNA during processing. Notably, the introns may play a role in gene regulation, but they do not remain in the mRNA that is translated into proteins. The entirety of the genome is not transcribed at once; only the gene that is 'turned on' or expressed is transcribed into mRNA.
Therefore, the correct answer to the question 'What size sections of the genome need to be transcribed?' is D) Gene-specific regions.