The alpha-helix and beta-pleated sheet are examples of which protein structure level?

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Secondary structure refers to regular, local patterns folded by the polypeptide backbone. The alpha-helix and beta-pleated sheet are classic examples because they are stabilized mainly by hydrogen bonds between backbone amide and carbonyl groups, not by side-chain interactions. This local folding defines how the chain temporarily adopts a helical or extended sheet form, independent of the exact amino acid sequence.

In contrast, the primary structure is simply the linear sequence of amino acids, the tertiary structure is the overall three-dimensional shape of a single polypeptide, and the quaternary structure describes how multiple polypeptide chains come together. So the alpha-helix and beta-pleated sheet exemplify secondary structure.