We usually prefer to use which when choosing between a limited number of options
(like two or three, or options from inside a category) and what when there is no
limit to the number of options:
▶️ Which do you like better – ketchup or mustard?
(only two options)
▶️ Which philosophy books do you recommend?
(options from inside the category of “philosophy books”)
▶️ What do you think about politics?
(there are infinite possible answers)
▶️ What’s wrong with the TV?
(there are infinite possible answers)
However, in spoken English, we often use “what” even when “which” would
technically be better:
▶️ I have black tea and green tea. What kind do you want?
The only time we MUST use “which” is when saying which of or which one(s):
▶️ Which of the students are going to fail the course?
▶️ Bob and Jerry, which one of you is going to help me fix the window?
▶️ All these cakes look delicious. I don’t know which one to buy!
▶️ I’m considering several different cars; I’m doing research to find out which
ones are most reliable.