The correct answer is "Officer, I have nothing illegal in the car."
If he then says, "Do you mind if I take a look in the car?" say "I do not consent to any search of my vehicle without a warrant."
If he pulled you over for a traffic infraction, then he is very limited in the extent of any search.
While protecting your 4th amandement rights, do not forget about exercising your 5th amandment rights.