$$\beta$$-pleated sheets of polypeptide chains are present in:

Correct option is A. secondary structure
The $$\beta$$-sheet (also $$\beta$$-pleated sheet) is a common moeity of regular secondary structure in proteins. Beta sheets consist of beta strands (also $$\beta$$-strand) connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet.