The correct option is C 30%
According to Chargaff's rule, [A] + [G] = [C] + [T].
However, Adenine binds to Thymine and Cytosine binds to Guanine. Therefore, [A] = [T], [G] = [C].
Total content of DNA = 100 %.
[A] + [T] + [G] + [C] = 100%
But, [T] = 20 % = [A]
20 % + 20 % + [G] + [C] = 100 %
[G] + [C] = 100 % - 40 % = 60 %
But, [G] = [C]
2[G] = 60 %
[G] = 30% = [C]