Length of one turn of DNA is

DNA has the double helical structure where the two strands are tightly coiled over each other. B- DNA has right-handed helix with the major and minor groove. The diameter of the DNA is 20 A${}^o$. The number of base pairs in one turn is 10 and the distance between base pairs is 3.4 A${}^o$. So the total length of one turn of DNA is 10 x 3.4 =  34 A${}^o$.

So, the correct answer is option A.