Correct option (d)480
Explanation :
According to Chargaff's ruIe, molar amount of adenine is equal to that of thymine and cytosine equals to guanine, A + G = T + C. So, a segment of DNA with 120 adenine base and 120 cytosine base will have same number of each thymine and guanine base (as, A = T and C = G), i.e. 120 thymine bases, 120 guanine bases, thus a total of 480 nucleotides.