Final answer:
The percentage of thymine in a DNA molecule with 30% adenine is 30%, since adenine pairs with thymine, making answer choice C (30%) correct.
Step-by-step explanation:
If 30% of the bases within a DNA molecule are adenine (A), then according to the base pairing rules of DNA, the percentage of thymine (T) will also be 30%. This is because adenine always pairs with thymine in DNA, forming base pairs. Consequently, if a DNA molecule has 30% adenine, it will also have an equal amount of thymine to pair with each adenine nucleotide.
Expanding on this, since there are four bases in DNA - adenine (A), thymine (T), cytosine (C), and guanine (G) - and adenine pairs with thymine, while cytosine pairs with guanine, the percentage of cytosine and guanine will make up the remaining 40% of the DNA molecule's composition (100% - 30% adenine - 30% thymine = 40%). If the DNA were symmetric in its base composition, cytosine and guanine would each account for 20% of the bases, however, the actual percentage can vary if there is an uneven GC content.
To answer the question, the percentage of thymine in a DNA molecule with 30% adenine is 30%, which corresponds to answer choice C.