Final answer:
The statement 'A+G = T+C' is not one of Chargaff's rules for DNA. Chargaff's rules specifically state only that the amount of adenine equals the amount of thymine and the amount of guanine equals the amount of cytosine.
Step-by-step explanation:
The student's question revolves around which statement is not one of Chargaff's rules for DNA. Chargaff's rules, formulated by Erwin Chargaff, state that in DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). Therefore, options a), c), and d) are correct descriptions of Chargaff's rules.
However, option b) A+G = T+C is not one of Chargaff's rules. While the total amount of purines (A and G) will equal the total amount of pyrimidines (T and C) because each base pair consists of one purine and one pyrimidine, this is not explicitly stated as one of Chargaff's rules. The confusion may arise because, in a double-stranded DNA molecule, the two strands are complementary; for each A on one strand, there is a T across from it on the other strand, and for each C, there is a G. However, Chargaff's rules refer specifically to the quantities of individual nitrogenous bases, not the sums of purines and pyrimidines.