Final answer:
A helical conformation is not a feature of β sheets; instead, that describes the α-helix. β sheets feature regular hydrogen bonding and can have either parallel or antiparallel arrangements.
Step-by-step explanation:
The feature not commonly observed in β sheets is C. A helical conformation. β sheets ('B-pleated sheets') exhibit regular hydrogen bonding between adjacent strands and can be organized in either parallel or antiparallel orientation. These sheets have a planar or pleated sheet-like arrangement of amino acid backbones, which are crucial for the structure of proteins such as enzymes and fibrous materials like silk. However, a helical conformation is characteristic of the α-helix, which is a different type of secondary protein structure where the amino acid chain twists to form a helix, stabilized by internal hydrogen bonds; this is not a feature of β sheets.