Haemoglobin of adult human  

• A

  is a protein with only primary structure.

• B

  is a protein with only one polypeptide chain showing tertiary structure.

• C

  is a protein with quaternary structure having one $\mathrm{\alpha }$ type and three $\mathrm{\beta }$ type subunits.

• D

  is a protein with quaternary structure having two $\mathrm{\alpha }$ type and two $\mathrm{\beta }$ type subunits.