The complementary base pairing between nitrogenous bases (A-T and G-C) of two antiparallel strands of a double stranded DNA molecule impart specific ratio of these bases. According to Chargaff’s rule in all cellular DNAs, regardless of the species, number of adenosine residues is equal to number of thymidine residues which means that A=T; and the number of guanosine residues is equal to the number of cytidine residues; G= C. Hence, that the sum of the purine residues equals the sum of the pyrimidine residues; i.e., A + G = T + C; this means that purine to pyrimidine (A+G/T+C) or pyrimidine to purine ratio (T+C/A+G) is constant for a species, options A is correct. But there is no 1:1 ratio between two sum of purines and pyrimidines bases; A+T/G+C or A+C/T+G do not hold true due to specific complementary base pairing between purines and pyrimidines. This makes options B, C and D wrong.