***WHAT vs. WHICH***

WHAT is used to ask when there are an unknown

number or infinite possibilities for an answer.

What is your favorite color? -- The answer could be anything. The question does not specify any options.

WHICH is used if you are are choosing between a more limited number of items, already defined.

Which color do you like, blue or green? -- The answer to this could be only blue or green, the options are already specified.