Nucleic acids are essential biomacromolecules of the living cells. They are polymer of nucleotides. Nucleotides comprises of three basic units nitrogenous base, pentose sugar and phosphate. There are two types of nitrogenous bases purines and pyrimidines. Adenine (A) and Guanine (G) are purines whereas Cytosine (C), Uracil (U) and Thymine (T) are pyrimidines. Erwin Chargaff proposed two rules which are termed as Chargaff's rules. These are applicable for double helix DNA/RNA structure. First rule, in any double stranded DNA the number of guanine units equals the number of cytosine units and the number of adenine units equals the number of thymine units. Second rule, the composition of DNA varies from one species to another. Thus, in dsDNA/ dsRNA: 1. The molar ratio of A to T equals to 1. Similarly the molar ratio of G to c equals to 1 2. The sum of purines (A and G) equals that of pyrimidines (C and T) 3.The percentage of C+G does not necessary equal the percentage of A+T 4. The ratio of G+C and A+T is constant for any species Therefore the ratio that is generally constant for a given species is G+C/A+T as explained by Chargaff's laws and the correct option is C