Final answer:
The feature not commonly observed in α helices is the extended and stretched conformation; α helices have a right-handed helical twist and are stabilized by hydrogen bonds between specific amino acids, creating a regular structure.
Step-by-step explanation:
The feature that is not commonly observed in α helices is C. Extended and stretched conformation. α helices are characterized by a right-handed helical twist, hydrogen bonds between the backbone amide hydrogen and carbonyl oxygen, and a regular repeating structure along the protein chain. These helices are formed as the polypeptide coils due to the intrachain hydrogen bonding between the carbonyl oxygen atom of one amino acid and the amide hydrogen atom four amino acids up the chain. In contrast, an extended and stretched conformation describes a structure more akin to β-pleated sheets than to the coiled nature of an α-helix.