Biochemists have found that though the amount of any given base found in DNA varies considerably from one species to another, the amount of adenine is always equal to the amount of:

Thymine

Explanation:

DNA is composed of four different nitrogenous bases called adenine, thymine, cytosine and guanine. The adenine and guanine belong to the purines whereas the cytosine and thymine belongs to the pyrimidines.

Although the amount of DNA varies in the organisms the pairing between two types of nitrogenous bases remains the same or it follows the complementary base pairing rule.

The complementary rule was given by Erwin Chargaff which stated that a purine always binds complementary pyrimidine on another strand. This shows that adenine binds thymine and guanine binds the cytosine.

So the amount of one adenine equals the thymine whereas the amount of guanine binds the cytosine.

Thus, thymine is the correct answer.