Answer:
C. Secondary
Step-by-step explanation:
In proteins, the structure of local segments can be arranged into three different three-dimensional formations: alpha helices (α-helix), beta sheets (β-strand) and omega loops. An alpha helix is the most common secondary protein conformation because it has low-energy and a stable arrangement. The α-helix is formed by the interaction between amino acids of every backbone N−H hydrogen bonded with the backbone C=O group of the corresponding amino acid residue in the polypeptide chain. The α-helix motif is very common in transmembrane segments of proteins that cross the lipid bilayer.