Which of the following caused the Sepoy Rebellion?

Which of the following caused the Sepoy Rebellion?

a. The British wanted Indian soldiers to wear boots that were made of animal skins.
b. The British wanted Indian soldiers to serve as personal servants to British royalty.
c. The British wanted Indian soldiers to use rifle cartridges that were greased with animal fat.
d. The British wanted Indian soldiers to take part in putting down Muslim rebellions.