The correct option is A 18
There are 10 base pair in one turn of DNA and two phosphodiester bonds are present between two adjacent base pairs,therefore there must be 9 phosphodiester bonds in one side of the 10 base pair and 9 phosphodiester bond on the other side.so the total number of phosphodiester bonds found in one turn of DNA is 18. so, 'the correct option is 18'.