Answer:a). base pairing rules
Explanation: A DNA is a double stranded helix in which the two DNA strands are antiparallel and complementary to each other. Watson and Crick developed the base pairing rules that adenine pairs with thymine and cytosine pairs with guanine. The complementary strand are not the same in base sequence and composition instead anywhere adenine is found in one strand, thymine is found in the other strand and anywhere cytosine is found in one strand, guanine occurs in the other strand.
