The right answer is phosphate.
DNA has a double helix structure (discovered in 1953 by James Dewey Watson, Francis Crick et al.).
DNA is a polymer of deoxyribonucleic bases, more commonly called nucleotides. Each nucleotide consists of a phosphate group (or phosphoric acid) bonded to a sugar, deoxyribose, itself linked to a nitrogenous base. These bases are four in number: adenine (denoted A), thymine (denoted T), cytosine (denoted C) and guanine (denoted G). The backbone of the DNA is formed by the sugar-phosphate repetition.