Final answer:
In double-stranded DNA, the base ratios of A/T and C/G always equal 1, reflecting the principle of complementary base pairing as per Chargaff's rules.
Step-by-step explanation:
For double-stranded DNA, the base ratio that ALWAYS equals 1 is C/G and A/T. This is based on the principle of complementary base pairing, where adenine (A) always pairs with thymine (T), and cytosine (C) pairs with guanine (G). The amount of adenine is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine in a DNA molecule. This concept is a fundamental principle explained by Chargaff's rules, which state that DNA from any cell of any organism exhibits a one-to-one ratio (1:1) of pyrimidine and purine bases (T and A, or C and G) and that the number of adenine units is approximately equal to the number of thymine units, and the number of guanine units is approximately equal to the number of cytosine units.
Therefore, the correct answer to the question is d) two of the above, as both A/T and C/G base ratios always equal 1 in double-stranded DNA.