The answer is thymine.
The base unit of DNA is called a nucleotide. The nucleotide consists of a nucleobase (adenine - A, thymine - T, cytosine - C, and guanine -G), a monosaccharide deoxyribose, and a phosphate group. Two nucleotides of the different strands are joined by hydrogen bonds. According to the base pairing rules, adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G).