The correct option is A Keratin
Based on the composition, proteins are classified into two categories - simple proteins and conjugated proteins.
Those proteins which are only made up of amino acids are called simple proteins, for example - keratin.
Whereas those proteins that are composed of aminoacids as well as other molecules are called conjugated proteins. For example - glycoprotein (carbohydrate and protein), nucleoprotein (nucleic acids and protein) and lipoprotein (lipid and protein).
Thus keratin is the odd one out in the given options as it is a simple protein, whereas the rest of the proteins given are conjugated proteins.