Final answer:
According to the base pairing rules in DNA, 15% of the DNA should be guanine if 15% is cytosine.
Step-by-step explanation:
The question relates to the base pairing rules in the structure of DNA. In DNA, Adenine (A) pairs with Thymine (T) and Cytosine (C) pairs with Guanine (G). This is referred to as Chargaff's Rule.
Therefore, if 15% of the DNA is composed of cytosine, an equal percentage, 15%, should be composed of guanine. So, the answer is a. 15%.
Learn more about DNA base pairing