The answer is letter C. The sides of the DNA molecule, the rails of the ladder, are composed of phosphates and sugars.
DNA strands constitute chemicals called nucleotides. DNA molecules on the other hand are made up of 2 polynucleotide chains. This polynucleotide is arranged on the backbone, the double helix. These nucleotides (polynucleotide and nucleotides) are composed of three parts: a phosphate, a sugar (deoxyribose), and a type of compound base. Deoxyribose and phosphate creates the backbone of the side of the ladder, nucleic acid while the base join both polynucleotide chains like the rungs of the ladder.