Proteins are organized into four levels of structure, each defined by how the chain is arranged and stabilized. The first level is the linear sequence of amino acids linked by peptide bonds, which provides the information that guides all folding. The local folded patterns—the alpha helices and beta sheets—make up the secondary structure and are held together mainly by hydrogen bonds between backbone atoms. The overall three-dimensional shape of the single polypeptide, or tertiary structure, arises from a balance of interactions: hydrophobic effects that bury nonpolar side chains, ionic interactions between charged groups, hydrogen bonds, and covalent disulfide bonds that can lock distant parts of the chain together. When several polypeptide chains associate into a functional unit, the quaternary structure describes that arrangement, stabilized by similar noncovalent forces and sometimes disulfide bonds between subunits. The option that lists primary as amino acid sequence, secondary as alpha helices and beta sheets held together by hydrogen bonds, tertiary as 3D folding stabilized by hydrophobic, ionic, hydrogen, and disulfide interactions, and quaternary as an assembly of multiple polypeptide subunits is the best because it matches the classical definitions and stabilizing forces for each level. The other descriptions mix up what each level is or what stabilizes it, such as tying secondary to sequence or claiming tertiary is limited to disulfide bonds, or describing quaternary structure as a single chain.

Three-dimensional folding driven by interactions among the side chains shapes a protein’s final form. The linear sequence of amino acids—the primary structure—sets up which side chains are where along the chain. Local patterns stabilized by backbone hydrogen bonds give rise to secondary structure, like alpha helices and beta sheets. But the overall 3D shape—the way the chain coils and folds so distant parts come into contact and the molecule becomes a compact, functional unit—is determined by how the side chains interact: hydrophobic collapsed regions hiding from water, ionic and hydrogen bonds between charged or polar groups, disulfide bridges between cysteines, and close packing from van der Waals forces. This integrated folding results in the tertiary structure, the specific three-dimensional arrangement that enables the protein’s function. If multiple polypeptide chains were involved, their arrangement into a functional complex would be the quaternary structure, not this single-chain folding.

RNA is built from four nitrogenous bases: adenine, cytosine, guanine, and uracil. Thymine is used in DNA, not RNA, so uracil takes thymine’s place in RNA. The set adenine, cytosine, guanine, and uracil matches RNA’s actual base composition, while options that include thymine or omit uracil do not fit.

Deoxyribose is the sugar in DNA. It’s a five-carbon sugar known as 2'-deoxyribose because it lacks the oxygen atom on the 2' carbon; RNA, by contrast, uses ribose, which has a hydroxyl group at that position. That absence of the 2'-OH in DNA makes the backbone more chemically stable. Fructose and glucose are hexoses used for energy, not part of the DNA backbone, and ribose is the sugar in RNA. So the sugar that forms DNA’s backbone is deoxyribose.