The force that holds DNA together in a double helix is:
(a) The force of the twist (b) Covalent bonds
(c) Ionic bonds or ionic interactions (d) Hydrogen bonds
The bases in two strands are paired through hydrogen bonds forming base pairs.
Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa.
Similarly, Guanine is bonded with Cytosine with three H-bonds.
As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix