(D) Hydrogen bond
The helical structure of proteins is stabilized by hydrogen bonds. In helical structure, the carbonyl oxygen atoms in C=O points in one direction, towards the amide hydrogen atoms in NH groups, 4 residues away. Together these groups form a hydrogen bond, one of the main forces of secondary structure stabilization in proteins. Hydrogen bonds are shown by dashed lines.
a- helix
The telephone cord shape of the a- helix is held in place by Hydrogen bonds between every N-H group and the oxygen of a C=O group in the next turn of the helix, four amino acids down the chain. The typical a- helix is about 11 amino acids long.
