Which of the following statements shows an example of operant conditioning?

I think it's - A chicken learns that if it pecks on a button, a food pellet will be released into its cage.

a. when a cat learns to drool at the sound of a can opener

b. when a dog refuses to play dead

c. when a cat and a dog share the same water bowl

d. when a dog plays dead she gets a treat in order to encourage her to repeat the behavior