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

The correct answer is Option (3) → 0.34 nm

In the double helix, the pitch of the helix is 3.4 nm, and there are about 10 base pairs in each turn. So the distance between two base pairs is 3.4 nm ÷ 10 = 0.34 nm.

NCERT Extract:
Structure of Polynucleotide Chain – “… (iv) 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.”