Final answer:
Based on Chargaff's rules and given that adenine content is 25%, the combined percentage of guanine and cytosine in Aspergillus DNA is also 50%, answering the student's question.
Step-by-step explanation:
The question from a student involves determining the percentage of guanine (G) and cytosine (C) in the DNA of Aspergillus given that the DNA has an adenine (A) content of 25%. This question relates to Chargaff's rules, which state that in DNA, the amount of adenine is equal to the amount of thymine (T), and the amount of guanine is equal to the amount of cytosine. Since the adenine content is 25%, it implies that the thymine content is also 25%. Together, A and T make up 50% of the DNA content. As DNA is composed of equal amounts of purines (A and G) and pyrimidines (T and C), the remaining 50% must be composed of guanine and cytosine combined. Therefore, the percentage of G + C within the Aspergillus DNA is 50%, which makes option C the correct answer.