Which of the following constitutes the primary structure of a protein?
a. the folding of a polypeptide chain
b. the linear sequence of amino acids in a polypeptide chair
c. the polypeptide chains stacked on top of each other
d. a pleated sheet
e. several polypeptide subunits

B. The linear sequence of amino acids in a polypeptide chain.

Explanation: Protein structure is the  three dimensional arrangement of amino acid molecules in polypeptide chain.  

Protein has four types of structure -

1-Primary structure

2-secondary structure

3- Tertiary structure

4- Quaternary structure

Primary structure of protein is the simplest form of protein where amino acids are arranged in linear way. One end of polypeptide chain is Called C-terminal (Carboxyl group )and other is called N terminal (amino group). Peptide bods play important role in establishing primary structure of protein.