According to Chargaff’s rule all cellular DNAs, regardless of the species, the number of adenosine residues is equal to the 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. According to the question, there are 120 adenine and 120 cytosine bases which mean the number of thymidine residues is 120 and that of guanosine residues is 120. DNA is the polymer of deoxyribonucleotides; each of which is made up of sugar, the nitrogenous base, and the phosphate moiety. Thus, the total number of nitrogenous bases represents the total number of nucleotides in a DNA chain. The total number of nucleotides in given DNA chain=A+T+G+C=480. Option A is the correct answer.