Step 1: Understanding DNA structure.
DNA consists of two antiparallel polynucleotide strands forming a double helix.
Step 2: Base pairing mechanism.
Complementary nitrogenous bases pair with each other through hydrogen bonds: adenine with thymine via two hydrogen bonds and guanine with cytosine via three hydrogen bonds.
Step 3: Eliminating other options.
Phospho-di-ester bonds join nucleotides within a strand, peptide bonds join amino acids, and glycosidic bonds link sugars to bases.
Step 4: Conclusion.
Thus, complementary nucleotides in DNA are held together by hydrogen bonds.