Which type of bond holds together the two strands of a DNA molecule?

The correct answer is hydrogen bonds because they are responsible for holding the two strands of a DNA molecule together in a double helix structure. Each strand of DNA is composed of a sugar-phosphate backbone that is linked by covalent bonds, but it is the hydrogen bonds that form between the complementary nitrogenous bases—adenine pairs with thymine via two hydrogen bonds, and cytosine pairs with guanine via three hydrogen bonds. This specific pairing and the nature of hydrogen bonding allow the strands to maintain a stable yet flexible structure that is critical for DNA's function in storing and transferring genetic information.

In contrast, ionic bonds involve electrostatic attraction between charged ions, which is not a feature in the stability of the double helix. Covalent bonds provide strength within each strand but do not link the strands to each other. Metallic bonds, which occur between metal atoms and involve a 'sea of electrons', are not relevant in the context of molecular biology. Thus, hydrogen bonds are the key interactions that enable the specific base pairing and maintain the overall structure of DNA.