Final answer:
Hydrogen bonds between base pairs are what maintain the double helix structure of DNA by connecting complementary bases across strands, allowing for stability and the ability to 'unzip' for replication.
Step-by-step explanation:
The bonds that are numerous enough to hold the two strands of a double-stranded DNA molecule tightly together, while individually weak, are hydrogen bonds between base pairs. These hydrogen bonds form between the complementary bases, which are adenine (A) pairing with thymine (T) and cytosine (C) pairing with guanine (G). The sugar-phosphate backbones of the DNA strands are connected by strong covalent phosphodiester bonds, but it is the hydrogen bonding between the bases which stabilizes the interior of the double helix structure.
The cumulative effect of millions of these hydrogen bonds effectively holds the two strands together. This allows for the double helix structure to be maintained while still enabling the strands to 'unzip' relatively easily during processes like DNA replication, where the strands serve as templates for creating new DNA molecules.
Therefore, the correct answer is B. Hydrogen bonds between base pairs.