Secondary structure of protein refers to

• A

  mainly denatured proteins and structure of prosthetic groups
• B

  three-dimensional structure, especially the bond between amino acid residues that are distinct from each other in the polypeptide chain
• C

  linear sequence of amino acid residues in the polypetide chain
• Correct answer
  D

  regular folding patterns of continous portions of the polypeptide chain

Hence, the correct answer is option: 'D'.