... and if your looking to pick up some slang, 'whatever' is commonly used as a one-word answer to any questions that offers a choice. Note: it does imply lack of caring (aka being apathetic) ex1. 'Do you want to go to the mall or the movies?' 'Whatever' ex2. 'Do you want this cookie or that cookie?" 'Whatever'

Not always. "Whichever" implies that there is some sort of choice involved. "Eat whichever cookie you want." But you could never say "Eat whichever want" if you mean "you can eat anything in the house." If you just say "Eat whichever you want" the context has to be that there are some restrictions on what the person can eat and they can choose which one they want. If you say "eat whatever you want" it means the person can eat anything in the house, there are no restrictions. But you can also say, "Eat whatever cookie you want." So you can always use "whatever" but only "whichever" in a context where there is a limited amount of things for the person to choose.