Answer: B. guanine
In a double helical structure of the DNA, the two strands of the DNA lies in a anti-parallel conformation. The complementary strands of the DNA follows the complementary base pairing rule given by Chargaff. It states that any cell of the organism should have 1:1 ratio of the pyrimidine and purine basis. The amount of thymine is equal to adenine and the amount of cytosine is to guanine. Adenine forms double hydrogen bond with Thymine and Cytosine forms triple hydrogen bond Guanine.