Answer: C - thymine
This is because of the complementary base pairing rule, where adenine always pairs with thymine, and guanine always pairs with cytosine. The amount of guanine in a double-stranded DNA molecule is always equal to the amount of cytosine, also due to this complementary base pairing rule.
Therefore, in a double-stranded DNA molecule, the amounts of adenine and thymine are always equal, as are the amounts of guanine and cytosine. This is known as Chargaff's rule, which states that the amount of purines (adenine and guanine) is equal to the amount of pyrimidines (thymine and cytosine) in a double-stranded DNA molecule.
