Final answer:
The strongest base pair is guanine-cytosine because it is held together by three hydrogen bonds, compared to the two hydrogen bonds in the adenine-thymine pair.
Step-by-step explanation:
The question asks which of the following is the strongest base pair. Base pairing in DNA involves the formation of bonds between complementary base pairs. Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). These pairs are held together by hydrogen bonds. Adenine and thymine form two hydrogen bonds, while guanine and cytosine form three hydrogen bonds. Thus, the strongest base pair due to the number of hydrogen bonds is guanine-cytosine. Therefore, the correct answer to the question is option B) guanine-cytosine.