Chargaff's rules primarily state that in any given double-stranded DNA, the amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C). Based on this understanding, the correct options that describe Chargaff's rules are:
B. The percentage of A is always equal to the percentage of T. And the percentage of G is almost always equal to the percentage of C.
This option directly reflects Chargaff's findings regarding base pair equivalency.
A. The DNA of all members of a species has the same nitrogenous base compositions. Although this statement may be true for individuals within a species, it does not describe Chargaff's rules specifically. Chargaff's rules apply more to the relationships between the bases rather than their percentages across a species.
C. For every adenine is a double-stranded DNA molecule. This statement is incomplete as it does not mention thymine, nor does it fully represent Chargaff's rules.
Therefore, the only correct selection that accurately describes Chargaff's rules is B.