Final answer:
The largest portion of the human genome is made up of noncoding DNA known as introns. They make up approximately 95% of nuclear DNA and typically are longer than exons, the coding sequences.
Step-by-step explanation:
The largest portion of the human genome is made up of noncoding DNA, which includes a substantial amount of introns. Introns are the noncoding sections within a gene that interrupt the coding sequences, which are known as exons. The human genome is predominantly composed of noncoding DNA that does not encode proteins or stable RNA products. While the exact function of much of this noncoding DNA is still not fully understood, it often plays a role in gene regulation and genome structure.
It is important to note that introns are not the same as junk DNA, which is a broader category that includes non-functional sequences such as pseudogenes and long stretches of DNA with no known purpose.
Thus, the answer to the question is introns (A), as they make up approximately 95% of human nuclear DNA and are generally much longer than the coding sequences, the exons.