What type of bonds hold the two anti-parallel strands of DNA together?

The two anti-parallel strands of DNA are held together primarily by hydrogen bonds. These bonds form between the nitrogenous bases of each strand, specifically between adenine and thymine (A-T) and between guanine and cytosine (G-C). Each base pair is stabilized by hydrogen bonds: A and T form two hydrogen bonds, while G and C form three. This base pairing is essential for the double helical structure of DNA, allowing for the complementary nature of the strands, which is critical for processes such as replication and transcription.

While there are covalent bonds present in the DNA backbone, which link the sugars and phosphates together, these do not bond the two strands to each other. Ionic bonds and hydrophobic interactions are also not involved in the direct association of the strands; ionic bonds can occur between charged molecules in other contexts, and hydrophobic interactions relate to the interactions of nonpolar regions in the structure. Therefore, the formation of the double helix structure and the stability of the base pairs are fundamentally reliant on hydrogen bonds, making this the correct answer.