The secondary structure of protein is stabilised by:

Proteins have four levels of structural organisation: primary, secondary, tertiary, and quaternary.

The primary structure is the linear sequence of amino acids held together by peptide bonds ($$-CO-NH-$$).

The secondary structure refers to the local folding patterns of the polypeptide chain, primarily the $$\alpha$$-helix and $$\beta$$-pleated sheet conformations. These structures are stabilised by hydrogen bonds formed between the $$C=O$$ group of one amino acid residue and the $$N-H$$ group of another amino acid residue in the polypeptide backbone.

Glycosidic bonds are found in carbohydrates, not proteins. Van der Waals forces contribute to the tertiary and quaternary structures but are not the primary stabilising force for the secondary structure.

Therefore, the secondary structure of a protein is stabilised by hydrogen bonding.