Which structural level of protein is considered the most complex?

Determine which of the following statements describes a tertiary structure of a protein.

Indicate whether each of the following statements describes a primary, secondary, tertiary, or quaternary protein structure:                                                            

______ a) Hydrophobic R groups seeking a nonpolar environment move toward the inside of the folded protein.  

______ b) Protein chains of collagen form a polypeptide chain composed of 3 alpha helices. 

______ c) An active protein contains 4 tertiary subunits. 

______ d) Two polypeptide chains held together by disulfide bridges. 

How many levels of protein structure are there?