Final answer:
The smallest portion of the human genome is made up of exons, which are significantly shorter when compared to introns and encompass about 5% of the genome.
Step-by-step explanation:
The type of DNA that makes up the smallest portion of the human genome is C) exons. Exons are the protein-coding portions of genes, and they are significantly shorter than introns, which are the noncoding portions. In the human genome, introns make up a much larger fraction; for instance, approximately 95% of a gene can be intronic whereas exons make up only about 5%. Introns may play roles in gene regulation and are removed from pre-mRNA during processing. The notion of introns as merely "junk DNA" is outdated, as they serve important evolutionary functions such as buffering mutations and allowing for exon shuffling, which can increase protein diversity and function.