Answer:
WHICH is your phone number?
Explanation:
I think that there is a grammar mistake in your exercise. You should have ¨______ is your phone number?¨ and not ¨_____It's your phone number?¨
Anyway, I´ll try to help you by giving you a correct sentence.
How many is ask to talk about quantity.
When is to ask about dates.
How is to ask about manner.
Which is ta ask about things or people.
Then, WHICH is the correct answer.
Transaltion:
¿Cuál es tu número de celular?
