-Keratin is an intermediate filament with a basic structural unit of two helices in a coiled coil. Each helix has a seven residue repeating unit...

α-Keratin is an intermediate filament with a basic structural unit of two α helices in a coiled coil. Each helix has a seven residue repeating unit (heptad repeat). A representation looking down the α helices of a coiled coil dimer is shown below. Each letter represents a different amino acid residue. 

three answers are correct and a and c are on the inside of the keritan molecule

(A) Keratin molecules are very strong due to hydrophilic interactions betweencharged amino acid side chains.