Answer:
"¿Cuál es tu número de teléfono?"
Explanation:
Assuming this person is asking a <em>friend, </em>they would want to keep things casual, hence the 'tu'.
One might ask: "¿Cuál es su número de teléfono?" If they were referring to a superior, or someone who should be treated with great respect.
(In this case, <em>su, </em>still means 'your' except in a more formal manner)
If you were to ask someone else what the telephone number of <em>someone else </em>is, you can also say the question "¿Cuál es su número de teléfono?"
(In this case, <em>su,</em> translates to: her or his)
