The bases are held together in a DNA double helix by hydrogen bonds. These bonds are

The correct option is Non-covalent bonds.

To understand why this is correct, let's break down the nature of the chemical bonds holding the DNA double helix together:
1. DNA Structure: The DNA double helix consists of two antiparallel strands of nucleotides. The backbone of each strand is formed by strong covalent phosphodiester bonds. However, the two strands are held together laterally across the helix by hydrogen bonds between complementary nitrogenous bases (adenine pairs with thymine, and guanine pairs with cytosine).
2. Nature of Hydrogen Bonds: A hydrogen bond is an attractive electrostatic interaction between a hydrogen atom covalently bonded to a highly electronegative atom (like nitrogen or oxygen) and another electronegative atom with a lone pair of electrons. Because it does not involve the sharing of electron pairs (which characterizes covalent bonds) or the complete transfer of electrons resulting in electrostatic attraction between oppositely charged ions (which characterizes ionic bonds), it is classified as a type of non-covalent interaction.
3. Importance of Non-covalent Bonds in DNA: Non-covalent bonds, including hydrogen bonds, are individually much weaker than covalent or ionic bonds. This relatively weak nature is biologically crucial because it allows the two strands of the DNA double helix to be easily zipped open (denatured) during DNA replication and transcription, and then zipped back together (annealed) without requiring large amounts of energy.