How are the bases in the two strands of DNA paired?

Hydrogen bonds

The correct answer is Option (2)- Hydrogen bonds

The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). 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. Hydrogen bonds are ideal for this purpose because they are strong enough to hold the DNA strands together but weak enough to allow the strands to be separated when necessary, such as during DNA replication or transcription.