Answer
      The correct option is primary structure.
Explantion
     A protein is composed of polypeptide chain which are organized into different structure as explain below:
Primary structure
      The primary structure of protein is composed of a single polypeptide chain in which amino acids are arranged in linear form. Only covalent bond (peptide bond) hold the structure.
Example
       Insulin hormone
Secondary structure
      In secondary structure a single polypeptide chain is arranged in beta pleated sheet or in helical form. hydrogen bond and covalent bond responsible for providing stability to structure.
Example
      Myoglobin
Tertiary structure
      In tertiary structure a single polypeptide chain is arranged in three dimension. covalent bond, hydrogen bond and disulfide linkage help in maintaning this structure.
Example
      Secretory proteins.
Quaternary structure
      It is composed of more than one polypeptide chain and arranged in three dimension forming a globular structure. covalent bond, hydrogen bond, disulfide linkage, hydrophobic and hydrophilic interaction help in maintaining this structure.
Example
      Haemoglobin.
