Final answer:
According to Chargaff's rules, in a DNA molecule, the percentage of thymine is equal to the percentage of adenine, so if adenine is 30%, thymine will also be 30%.
Step-by-step explanation:
If 30% of the bases within a DNA molecule are adenine, according to Chargaff's rules, the percentage of thymine would also be 30%. Chargaff's rule states that DNA is composed of base pairs, and the amount of adenine (A) will always equal the amount of thymine (T), while the amount of guanine (G) equals the amount of cytosine (C). Therefore, in a DNA molecule where adenine is 30%, thymine would also be 30%, which answers the multiple-choice question with option B. 30%.