What will be the percentage of guanine in a DNA molecule having 20% adenine ?

The correct option is B 30%
According to the Chargaff's rules, the DNA has 1:1 ratio of purine bases (cytosine and thymine) and pyrimidine bases (guanine and adenine). This is called as the base pair rules. It means the amount of guanine will be equal to cytosine and the amount of adenine will be equal to thymine. In the given problem, adenine is 20%. Hence, thymine will be 20%. So the percentage of cytosine and guanine will be 30% each.

Thus, the correct answer is option B.