Which word is abstract?

garden
friendship
airplane
sunshine

One historical reason that people in the indian subcontinent have been reluctant to use livestock for food has been that a. their digestive systems are unable to handle meat and proteins b. they refused to eat meat in order to show resistance to british imperialism c. they are able to use cattle as a labor mainstay of their agriculture d. they can use the meat from cattle as fertilizer to make crops grow better