The correct answer is option 3. "secondary structure".
Explanation:
The secondary structure of polypeptide and proteins refers to the second order of folding, including common structures such as beta sheets, alpha helix, as well as turns and loops. The hydrogen bonds between the oxygen and nitrogen atoms of the peptide bonds are crucial to secondary structures, as these hydrogen bonds form and stabilize the aforementioned common structures.
Diffusion has a membrane which allows even large molecules to pass through it whereas osmosis allows only water molecules as it has a partially permeable membrane.
