Explanation:

The double helix of DNA is composed of two polynucleotide chains. The two chains are held together by hydrogen bonds between specific pairs of bases: A-T and G-C. Adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). This specific base pairing is called complementary base pairing and is essential for the structure and function of DNA.

The other options, covalent bonds, ionic bonds, and van der Waals forces, are all incorrect. Covalent bonds are strong chemical bonds that hold atoms together within a molecule. Ionic bonds are electrostatic attractions between positively and negatively charged ions. Van der Waals forces are weak attractions between nonpolar molecules. None of these types of bonds are involved in holding DNA strands together.