Which
Definition 1 of 4
Pro Tip 1/3
Choice Clarification
Use 'which' to clarify a choice from a limited set of options. There are several colors available, which do you prefer?
Pro Tip 2/3
Question Precision
When asking questions, 'which' helps to show you're expecting specific information as an answer. Which of these dishes is the spiciest?
Pro Tip 3/3
Not 'What'
'Which' implies a choice among a specific group, unlike the more general 'what'. There are many books here; which did you read?