Which part of DNA forms its structural backbone?

The structural backbone of DNA is formed by alternating sugar and phosphate groups. In the case of DNA, the sugar component is deoxyribose. The phosphate groups link the 5' carbon atom of one deoxyribose sugar to the 3' carbon of the next, creating a long, stable chain that provides structure and support to the molecule. This sugar-phosphate backbone serves as the framework that holds the genetic material together, supporting the attachment of nitrogenous bases (adenine, thymine, cytosine, and guanine) which extend from the backbone and are involved in base pairing and the encoding of genetic information.

Other combinations mentioned in the choices do not fulfill the requirement for a structural backbone. For example, although nitrogenous bases are part of DNA, they do not contribute to the backbone's structure but instead project from it. Furthermore, ribose is not found in DNA; it is the sugar component of RNA. Thus, those options do not accurately reflect the components that form DNA's structural backbone.