Final answer:
C and G base pairs are stronger than A and T or A and U pairs because they form three hydrogen bonds, which is one more than the two formed between A and T or A and U pairs, thus requiring more energy to separate them.
Step-by-step explanation:
The C and G base pairs are stronger than A and T (or A and U in RNA) pairs because they form a greater number of hydrogen bonds. Specifically, cytosine (C) and guanine (G) form three hydrogen bonds, while adenine (A) and thymine (T) or adenine (A) and uracil (U) form only two hydrogen bonds. The additional hydrogen bond means that more energy is required to separate C-G pairs, thus making them stronger.
Although covalent bonds are generally stronger than hydrogen bonds, within the structure of DNA, it's the pattern of hydrogen bonding between the bases that determines the stability of the base pairs. Hydrogen bonds are not as strong as covalent bonds but are significant in biological systems because of their ubiquity and their contribution to the stability and properties of biomolecules, such as DNA and proteins. In summary, the correct answer to the question is A) Greater number of hydrogen bonds.