Adenine on one strand can pair only with thymine on the other strand; cytosine, only with guanine.
Explanation:
DNA nucleotides can contain one of four nitrogenous bases. These bases are adenine (A), thymine (T), guanine (G) and cytosine (C). The two strands are held together through hydrogen bonds that form between the nitrogenous bases. Adenine (A) forms bonds with thymine (T) while cytosine (C) forms bonds with guanine (G); A only ever pairs with T, and C only ever pairs with G.
