Answer:
The correct answer is a. "exon; intron".
Step-by-step explanation:
According to the article "Distributions of exons and introns in the human genome" published in the scientific journal "In silico biology", an average human gene have 8.8 exons and 7.8 introns. Regardless of having similar proportions, human exons and introns differ in their lenght. An average exon's length is about 170 bp, when the average intron's length is 5419 bp. Therefore, most of the genetic material is comprised of introns (90%), while only a small part is comprised of exons (10%).