What Is the Best Way to Refuse to Serve Minors

What is the best way to refuse to serve minors (or any customer without proper identification) alcohol?

A) Threaten to call the police if they do not leave the establishment immediately.
B) Ignore them completely.
C) Politely tell them you cannot serve them and do not try to embarrass them further.
D) Tell them you can't serve them and immediately have them escorted off the premises by security.