The distance between a base pair (bp) in a DNA helix is approximately:

0.34 nm

The correct answer is Option (3) → 0.34 nm 

The DNA molecule consists of two polynucleotide chains that form a double helix structure. The DNA double helix is a right-handed helix, meaning the strands twist around each other in a right-handed manner along a common axis.The Double-helix structure of DNA  is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside. The two chains have anti-parallel polarity. It means, if one chain has the polarity 5'-3', the other has 3'-5'. 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 . The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm (a nanometre is one billionth of a metre, that is 10-9 m) and there are roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately 0.34 nm.