Final answer:
Hypervariable regions in genes are flanked by less variable sequences called exons, which are expressed protein-coding sequences in eukaryotic genes.
Step-by-step explanation:
Hypervariable regions are flanked by much less variable sequences known as exons. Exons are protein-coding sequences within genes that are expressed, while introns are non-coding sequences found in eukaryotic genes that are removed during the RNA splicing process to form mature mRNA. The correct answer to the question is a) Exons. In the context of immunoglobulins, hypervariable regions, also called complementarity determining regions (CDRs), are where the variability in amino acid sequence is the greatest, allowing antibodies to bind to a vast array of antigens. These regions are stabilized and supported by more conserved framework regions that make up the rest of the protein, most of which are exons.